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Be sure that your application is listening on the port provided by the PORT environment variable: process.env.PORT.
Only port 80 and 8080 is accessible (exposed ports). When your application fails to start, you can check the Docker logs in the LogFiles directory. To log the stdout and stderr from your container, you need to enable Application Logging under App Service Logs. The setting takes effect immediately. App Service detects the change and restarts the container automatically.
Based on the application framework, kindly you could startup command is configured - https://learn.microsoft.com/azure/app-service/containers/app-service-linux-faq#built-in-images
You can also always inspect the log files from the browser at https://<app-name>.scm.azurewebsites.net/api/logs/docker and leverage App Service diagnostics to fetch more details on the issue. To access App Service diagnostics, navigate to your App Service app in the Azure portal. In the left navigation, click on Diagnose and solve problems- From there, run the “Availability & Performance” & “Diagnostic Tools” for investigation.
Kindly let us know if this helps or you need further assistance, we would more than happy to assist you.