Hi @Alex Ross,
Welcome on Microsoft Q&A.
Let me answer your questions. Container instances allows you to setup one container from image stored in container registry but also you can deploy multi-container environment by using container group. An example: https://learn.microsoft.com/en-us/azure/container-instances/container-instances-multi-container-yaml?source=recommendations
All scenarios described in your question are possible.
More information: https://learn.microsoft.com/en-us/azure/container-instances/ Please upvote or accept as answer if it helped.