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Containerize a .NET app with dotnet publish

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Containers have many features and benefits, such as being an immutable infrastructure, providing a portable architecture, and enabling scalability. The image can be used to create containers for your local development environment, private cloud, or public cloud. In this tutorial, you learn how to containerize a .NET application using the dotnet publish command without the use of a Dockerfile. Additionally, you explore how to configure the container image and execution, and how to clean up resources.

Prerequisites

Install the following prerequisites:

In addition to these prerequisites, it's recommended that you're familiar with Worker Services in .NET.

Create .NET app

You need a .NET app to containerize, so start by creating a new app from a template. Open your terminal, create a working folder (sample-directory) if you haven't already, and change directories so that you're in it. In the working folder, run the following command to create a new project in a subdirectory named Worker:

dotnet new worker -o Worker -n DotNet.ContainerImage

Your folder tree looks like the following:

📁 sample-directory
    └──📂 Worker
        ├──appsettings.Development.json
        ├──appsettings.json
        ├──DotNet.ContainerImage.csproj
        ├──Program.cs
        ├──Worker.cs
        └──📂 obj
            ├── DotNet.ContainerImage.csproj.nuget.dgspec.json
            ├── DotNet.ContainerImage.csproj.nuget.g.props
            ├── DotNet.ContainerImage.csproj.nuget.g.targets
            ├── project.assets.json
            └── project.nuget.cache

The dotnet new command creates a new folder named Worker and generates a worker service that, when run, logs a message every second. From your terminal session, change directories and navigate into the Worker folder. Use the dotnet run command to start the app.

dotnet run
Building...
info: DotNet.ContainerImage.Worker[0]
      Worker running at: 10/18/2022 08:56:00 -05:00
info: Microsoft.Hosting.Lifetime[0]
      Application started. Press Ctrl+C to shut down.
info: Microsoft.Hosting.Lifetime[0]
      Hosting environment: Development
info: Microsoft.Hosting.Lifetime[0]
      Content root path: .\Worker
info: DotNet.ContainerImage.Worker[0]
      Worker running at: 10/18/2022 08:56:01 -05:00
info: DotNet.ContainerImage.Worker[0]
      Worker running at: 10/18/2022 08:56:02 -05:00
info: DotNet.ContainerImage.Worker[0]
      Worker running at: 10/18/2022 08:56:03 -05:00
info: Microsoft.Hosting.Lifetime[0]
      Application is shutting down...
Attempting to cancel the build...

The worker template loops indefinitely. Use the cancel command Ctrl+C to stop it.

Add NuGet package

Starting with .NET SDK version 8.0.200, the PublishContainer target is available for every project. To avoid depending on the Microsoft.NET.Build.Containers NuGet package, ensure that you're using the latest .NET SDK version. Additionally, your project file needs to have IsPublishable set to true and enable SDK container support.

Important

By default, the IsPublishable property is set to true for console, webapp, and worker templates.

To enable SDK container support, set the EnableSdkContainerSupport property to true in your project file.

<PropertyGroup>
  <IsPublishable>true</IsPublishable>
  <EnableSdkContainerSupport>true</EnableSdkContainerSupport>
</PropertyGroup>

Set the container image name

There are various configuration options available when publishing an app as a container.

By default, the container image name is the AssemblyName of the project. If that name is invalid as a container image name, you can override it by specifying a ContainerRepository as shown in the following project file:

<Project Sdk="Microsoft.NET.Sdk.Worker">

  <PropertyGroup>
    <TargetFramework>net8.0</TargetFramework>
    <Nullable>enable</Nullable>
    <ImplicitUsings>enable</ImplicitUsings>
    <UserSecretsId>dotnet-DotNet.ContainerImage-2e40c179-a00b-4cc9-9785-54266210b7eb</UserSecretsId>
    <ContainerRepository>dotnet-worker-image</ContainerRepository>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.Extensions.Hosting" Version="8.0.1" />
  </ItemGroup>
</Project>

For more information, see ContainerRepository.

Publish .NET app

To publish the .NET app as a container, use the following dotnet publish command:

dotnet publish --os linux --arch x64 /t:PublishContainer

The preceding .NET CLI command publishes the app as a container:

  • Targeting Linux as the OS (--os linux).
  • Specifying an x64 architecture (--arch x64).

Important

To build the container locally, you must have the Docker daemon running. If it isn't running when you attempt to publish the app as a container, you'll experience an error similar to the following:

..\build\Microsoft.NET.Build.Containers.targets(66,9): error MSB4018:
   The "CreateNewImage" task failed unexpectedly. [..\Worker\DotNet.ContainerImage.csproj]

The command produces output similar to the example output:

Determining projects to restore...
  All projects are up-to-date for restore.
  DotNet.ContainerImage -> .\Worker\bin\Release\net8.0\linux-x64\DotNet.ContainerImage.dll
  DotNet.ContainerImage -> .\Worker\bin\Release\net8.0\linux-x64\publish\
  Building image 'dotnet-worker-image' with tags latest on top of base image mcr.microsoft.com/dotnet/aspnet:8.0
  Pushed container 'dotnet-worker-image:latest' to Docker daemon

This command compiles your worker app to the publish folder and pushes the container to your local docker registry.

Configure container image

You can control many aspects of the generated container through MSBuild properties. In general, if you can use a command in a Dockerfile to set some configuration, you can do the same via MSBuild.

Note

The only exceptions to this are RUN commands. Due to the way containers are built, those cannot be emulated. If you need this functionality, you'll need to use a Dockerfile to build your container images.

ContainerArchiveOutputPath

Starting in .NET 8, you can create a container directly as a tar.gz archive. This feature is useful if your workflow isn't straightforward and requires that you, for example, run a scanning tool over your images before pushing them. Once the archive is created, you can move it, scan it, or load it into a local Docker toolchain.

To publish to an archive, add the ContainerArchiveOutputPath property to your dotnet publish command, for example:

dotnet publish \
  -p PublishProfile=DefaultContainer \
  -p ContainerArchiveOutputPath=./images/sdk-container-demo.tar.gz

You can specify either a folder name or a path with a specific file name. If you specify the folder name, the filename generated for the image archive file will be $(ContainerRepository).tar.gz. These archives can contain multiple tags inside them, only as single file is created for all ContainerImageTags.

Container image naming configuration

Container images follow a specific naming convention. The name of the image is composed of several parts, the registry, optional port, repository, and optional tag and family.

REGISTRY[:PORT]/REPOSITORY[:TAG[-FAMILY]]

For example, consider the fully qualified mcr.microsoft.com/dotnet/runtime:8.0-alpine image name:

  • mcr.microsoft.com is the registry (and in this case represents the Microsoft container registry).
  • dotnet/runtime is the repository (but some consider this the user/repository).
  • 8.0-alpine is the tag and family (the family is an optional specifier that helps disambiguate OS packaging).

Some properties described in the following sections correspond to managing parts of the generated image name. Consider the following table that maps the relationship between the image name and the build properties:

Image name part MSBuild property Example values
REGISTRY[:PORT] ContainerRegistry mcr.microsoft.com:443
PORT ContainerPort :443
REPOSITORY ContainerRepository dotnet/runtime
TAG ContainerImageTag 8.0
FAMILY ContainerFamily -alpine

The following sections describe the various properties that can be used to control the generated container image.

ContainerBaseImage

The container base image property controls the image used as the basis for your image. By default, the following values are inferred based on the properties of your project:

  • If your project is self-contained, the mcr.microsoft.com/dotnet/runtime-deps image is used as the base image.
  • If your project is an ASP.NET Core project, the mcr.microsoft.com/dotnet/aspnet image is used as the base image.
  • Otherwise the mcr.microsoft.com/dotnet/runtime image is used as the base image.

The tag of the image is inferred to be the numeric component of your chosen TargetFramework. For example, a project targeting net6.0 results in the 6.0 tag of the inferred base image, and a net7.0-linux project uses the 7.0 tag, and so on.

If you set a value here, you should set the fully qualified name of the image to use as the base, including any tag you prefer:

<PropertyGroup>
    <ContainerBaseImage>mcr.microsoft.com/dotnet/runtime:8.0</ContainerBaseImage>
</PropertyGroup>

Starting with .NET SDK version 8.0.200, the ContainerBaseImage inference has been improved to optimize the size and security:

  • Targeting the linux-musl-x64 or linux-musl-arm64 Runtime Identifiers, automatically chooses the alpine image variants to ensure your project runs:
    • If the project uses PublishAot=true then the nightly/runtime-deps jammy-chiseled-aot variant of the base image for best size and security.
    • If the project uses InvariantGlobalization=false then the -extra variants is used to ensure localization still works.

For more information regarding the image variants sizes and characteristics, see .NET 8.0 Container Image Size Report.

ContainerFamily

Starting with .NET 8, you can use the ContainerFamily MSBuild property to choose a different family of Microsoft-provided container images as the base image for your app. When set, this value is appended to the end of the selected TFM-specific tag, changing the tag provided. For example, to use the Alpine Linux variants of the .NET base images, you can set ContainerFamily to alpine:

<PropertyGroup>
    <ContainerFamily>alpine</ContainerFamily>
</PropertyGroup>

The preceding project configuration results in a final tag of 8.0-alpine for a .NET 8-targeting app.

This field is free-form, and often can be used to select different operating system distributions, default package configurations, or any other flavor of changes to a base image. This field is ignored when ContainerBaseImage is set. For more information, see .NET container images.

ContainerRuntimeIdentifier

The container runtime identifier property controls the operating system and architecture used by your container if your ContainerBaseImage supports more than one platform. For example, the mcr.microsoft.com/dotnet/runtime image currently supports linux-x64, linux-arm, linux-arm64 and win10-x64 images all behind the same tag, so the tooling needs a way to be told which of these versions you intend to use. By default, this is set to the value of the RuntimeIdentifier that you chose when you published the container. This property rarely needs to be set explicitly - instead use the -r option to the dotnet publish command. If the image you've chosen doesn't support the RuntimeIdentifier you've chosen, results in an error that describes the RuntimeIdentifiers the image does support.

You can always set the ContainerBaseImage property to a fully qualified image name, including the tag, to avoid needing to use this property at all.

<PropertyGroup>
    <ContainerRuntimeIdentifier>linux-arm64</ContainerRuntimeIdentifier>
</PropertyGroup>

For more information regarding the runtime identifiers supported by .NET, see RID catalog.

ContainerRegistry

The container registry property controls the destination registry, the place that the newly created image will be pushed to. By default it's pushed to the local Docker daemon, but you can also specify a remote registry. When using a remote registry that requires authentication, you authenticate using the well-known docker login mechanisms. For more information, See authenticating to container registries for more details. For a concrete example of using this property, consider the following XML example:

<PropertyGroup>
    <ContainerRegistry>registry.mycorp.com:1234</ContainerRegistry>
</PropertyGroup>

This tooling supports publishing to any registry that supports the Docker Registry HTTP API V2. This includes the following registries explicitly (and likely many more implicitly):

For notes on working with these registries, see the registry-specific notes.

ContainerRepository

The container repository is the name of the image itself, for example, dotnet/runtime or my-app. By default, the AssemblyName of the project is used.

<PropertyGroup>
    <ContainerRepository>my-app</ContainerRepository>
</PropertyGroup>

Image names consist of one or more slash-delimited segments, each of which can only contain lowercase alphanumeric characters, periods, underscores, and dashes, and must start with a letter or number. Any other characters result in an error being thrown.

ContainerImageTag(s)

The container image tag property controls the tags that are generated for the image. To specify a single tag use ContainerImageTag and for multiple tags use ContainerImageTags.

Important

When you use ContainerImageTags, you'll end up with multiple images, one per unique tag.

Tags are often used to refer to different versions of an app, but they can also refer to different operating system distributions, or even different configurations.

Starting with .NET 8, when a tag isn't provided the default is latest.

To override the default, specify either of the following:

<PropertyGroup>
    <ContainerImageTag>1.2.3-alpha2</ContainerImageTag>
</PropertyGroup>

To specify multiple tags, use a semicolon-delimited set of tags in the ContainerImageTags property, similar to setting multiple TargetFrameworks:

<PropertyGroup>
    <ContainerImageTags>1.2.3-alpha2;latest</ContainerImageTags>
</PropertyGroup>

Tags can only contain up to 127 alphanumeric characters, periods, underscores, and dashes. They must start with an alphanumeric character or an underscore. Any other form results in an error being thrown.

Note

When using ContainerImageTags, the tags are delimited by a ; character. If you're calling dotnet publish from the command line (as is the case with most CI/CD environments), you'll need to outer wrap the values in a single ' and inner wrap with double quotes ", for example (='"tag-1;tag-2"'). Consider the following dotnet publish command:

dotnet publish -p ContainerImageTags='"1.2.3-alpha2;latest"'

This results in two images being generated: my-app:1.2.3-alpha2 and my-app:latest.

Tip

If you experience issues with the ContainerImageTags property, consider scoping an environment variable ContainerImageTags instead:

ContainerImageTags='1.2.3;latest' dotnet publish

ContainerLabel

The container label adds a metadata label to the container. Labels have no impact on the container at run time, but are often used to store version and authoring metadata for use by security scanners and other infrastructure tools. You can specify any number of container labels.

The ContainerLabel node has two attributes:

  • Include: The key of the label.
  • Value: The value of the label (this may be empty).
<ItemGroup>
    <ContainerLabel Include="org.contoso.businessunit" Value="contoso-university" />
</ItemGroup>

For a list of labels that are created by default, see default container labels.

Configure container execution

To control the execution of the container, you can use the following MSBuild properties.

ContainerWorkingDirectory

The container working directory node controls the working directory of the container, the directory that commands are executed within if not other command is run.

By default, the /app directory value is used as the working directory.

<PropertyGroup>
    <ContainerWorkingDirectory>/bin</ContainerWorkingDirectory>
</PropertyGroup>

ContainerPort

The container port adds TCP or UDP ports to the list of known ports for the container. This enables container runtimes like Docker to map these ports to the host machine automatically. This is often used as documentation for the container, but can also be used to enable automatic port mapping.

The ContainerPort node has two attributes:

  • Include: The port number to expose.
  • Type: Defaults to tcp, valid values are either tcp or udp.
<ItemGroup>
    <ContainerPort Include="80" Type="tcp" />
</ItemGroup>

Starting with .NET 8, the ContainerPort is inferred when not explicitly provided based on several well-known ASP.NET environment variables:

  • ASPNETCORE_URLS
  • ASPNETCORE_HTTP_PORTS
  • ASPNETCORE_HTTPS_PORTS

If these environment variables are present, their values are parsed and converted to TCP port mappings. These environment variables are read from your base image, if present, or from the environment variables defined in your project through ContainerEnvironmentVariable items. For more information, see ContainerEnvironmentVariable.

ContainerEnvironmentVariable

The container environment variable node allows you to add environment variables to the container. Environment variables are accessible to the app running in the container immediately, and are often used to change the run-time behavior of the running app.

The ContainerEnvironmentVariable node has two attributes:

  • Include: The name of the environment variable.
  • Value: The value of the environment variable.
<ItemGroup>
  <ContainerEnvironmentVariable Include="LOGGER_VERBOSITY" Value="Trace" />
</ItemGroup>

For more information, see .NET environment variables.

Configure container commands

By default, the container tools launch your app using either the generated AppHost binary for your app (if your app uses an AppHost), or the dotnet command plus your app's DLL.

However, you can control how your app is executed by using some combination of ContainerAppCommand, ContainerAppCommandArgs, ContainerDefaultArgs, and ContainerAppCommandInstruction.

These different configuration points exist because different base images use different combinations of the container ENTRYPOINT and COMMAND properties, and you want to be able to support all of them. The defaults should be useable for most apps, but if you want to customize your app launch behavior you should:

  • Identify the binary to run and set it as ContainerAppCommand
  • Identify which arguments are required for your application to run and set them as ContainerAppCommandArgs
  • Identify which arguments (if any) are optional and could be overridden by a user and set them as ContainerDefaultArgs
  • Set ContainerAppCommandInstruction to DefaultArgs

For more information, see the following configuration items.

ContainerAppCommand

The app command configuration item is the logical entry point of your app. For most apps, this is the AppHost, the generated executable binary for your app. If your app doesn't generate an AppHost, then this command will typically be dotnet <your project dll>. These values are applied after any ENTRYPOINT in your base container, or directly if no ENTRYPOINT is defined.

The ContainerAppCommand configuration has a single Include property, which represents the command, option, or argument to use in the entrypoint command:

<ItemGroup Label="ContainerAppCommand Assignment">
  <!-- This is how you would start the dotnet ef tool in your container -->
  <ContainerAppCommand Include="dotnet" />
  <ContainerAppCommand Include="ef" />

  <!-- This shorthand syntax means the same thing, note the semicolon separating the tokens. -->
  <ContainerAppCommand Include="dotnet;ef" />
</ItemGroup>

ContainerAppCommandArgs

This app command args configuration item represents any logically required arguments for your app that should be applied to the ContainerAppCommand. By default, none are generated for an app. When present, the args are applied to your container when it's run.

The ContainerAppCommandArgs configuration has a single Include property, which represents the option or argument to apply to the ContainerAppCommand command.

<ItemGroup>
  <!-- Assuming the ContainerAppCommand defined above,
       this would be the way to force the database to update.
  -->
  <ContainerAppCommandArgs Include="database" />
  <ContainerAppCommandArgs Include="update" />

  <!-- This is the shorthand syntax for the same idea -->
  <ContainerAppCommandArgs Include="database;update" />
</ItemGroup>

ContainerDefaultArgs

This default args configuration item represents any user-overridable arguments for your app. This is a good way to provide defaults that your app might need to run in a way that makes it easy to start, yet still easy to customize.

The ContainerDefaultArgs configuration has a single Include property, which represents the option or argument to apply to the ContainerAppCommand command.

<ItemGroup>
  <!-- Assuming the ContainerAppCommand defined above,
       this would be the way to force the database to update.
  -->
  <ContainerDefaultArgs Include="database" />
  <ContainerDefaultArgs Include="update" />

  <!-- This is the shorthand syntax for the same idea -->
  <ContainerDefaultArgs Include="database;update" />
</ItemGroup>

ContainerAppCommandInstruction

The app command instruction configuration helps control the way the ContainerEntrypoint, ContainerEntrypointArgs, ContainerAppCommand, ContainerAppCommandArgs, and ContainerDefaultArgs are combined to form the final command that is run in the container. This depends greatly on if an ENTRYPOINT is present in the base image. This property takes one of three values: "DefaultArgs", "Entrypoint", or "None".

  • Entrypoint:
    • In this mode, the entrypoint is defined by ContainerAppCommand, ContainerAppCommandArgs, and ContainerDefaultArgs.
  • None:
    • In this mode, the entrypoint is defined by ContainerEntrypoint, ContainerEntrypointArgs, and ContainerDefaultArgs.
  • DefaultArgs:
    • This is the most complex mode—if none of the ContainerEntrypoint[Args] items are present, the ContainerAppCommand[Args] and ContainerDefaultArgs are used to create the entrypoint and command. The base image entrypoint for base images that have it hard-coded to dotnet or /usr/bin/dotnet is skipped so that you have complete control.
    • If both ContainerEntrypoint and ContainerAppCommand are present, then ContainerEntrypoint becomes the entrypoint, and ContainerAppCommand becomes the command.

Note

The ContainerEntrypoint and ContainerEntrypointArgs configuration items have been deprecated as of .NET 8.

Important

This is for advanced users-most apps shouldn't need to customize their entrypoint to this degree. For more information and if you'd like to provide use cases for your scenarios, see GitHub: .NET SDK container builds discussions.

ContainerUser

The user configuration property controls the default user that the container runs as. This is often used to run the container as a non-root user, which is a best practice for security. There are a few constraints for this configuration to be aware of:

  • It can take various forms—username, linux user ids, group name, linux group id, username:groupname, and other ID variants.
  • There's no verification that the user or group specified exists on the image.
  • Changing the user can alter the behavior of the app, especially in regards to things like File System permissions.

The default value of this field varies by project TFM and target operating system:

  • If you're targeting .NET 8 or higher and using the Microsoft runtime images, then:
    • on Linux the rootless user app is used (though it's referenced by its user ID)
    • on Windows the rootless user ContainerUser is used
  • Otherwise, no default ContainerUser is used
<PropertyGroup>
  <ContainerUser>my-existing-app-user</ContainerUser>
</PropertyGroup>

Tip

The APP_UID environment variable is used to set user information in your container. This value can come from environment variables defined in your base image (like that Microsoft .NET images do), or you can set it yourself via the ContainerEnvironmentVariable syntax.

To configure your app to run as a root user, set the ContainerUser property to root. In your project file, add the following:

<PropertyGroup>
  <ContainerUser>root</ContainerUser>
</PropertyGroup>

Alternatively, you can set this value when calling dotnet publish from the command line:

dotnet publish -p ContainerUser=root

Default container labels

Labels are often used to provide consistent metadata on container images. This package provides some default labels to encourage better maintainability of the generated images.

  • org.opencontainers.image.created is set to the ISO 8601 format of the current UTC DateTime.

For more information, see Implement conventional labels on top of existing label infrastructure.

Clean up resources

In this article, you published a .NET worker as a container image. If you want, delete this resource. Use the docker images command to see a list of installed images.

docker images

Consider the following example output:

REPOSITORY             TAG       IMAGE ID       CREATED          SIZE
dotnet-worker-image    1.0.0     25aeb97a2e21   12 seconds ago   191MB

Tip

Image files can be large. Typically, you would remove temporary containers you created while testing and developing your app. You usually keep the base images with the runtime installed if you plan on building other images based on that runtime.

To delete the image, copy the image ID and run the docker image rm command:

docker image rm 25aeb97a2e21

Next steps