Custom Dockerfile support in PublishContainer target

[dombrovsky]

We are using custom Dockerfiles for building container images for our csprojs. We are also using .NET Aspire, and we would like to use Aspire8 project to generate helm yamls.
Aspir8 project relies on dotnet publish -t:PublishContainer to build images for csprojs, which ignores existing custom Dockerfile.

A feature request is to make PublishContainer target respect DockerfileFile property, so that when it is specified dotnet publish -t:PublishContainer command would build image using specified Dockerfile.

Alternative solution would be to add support to Apire8 for calling docker build instead of dotnet publish. However it feels that having it in dotnet sdk could be useful in other scenarios as well.

[baronfel]

Hello - we track feature requests for containers over at https://github.com/dotnet/sdk-container-builds - specifically this request is /issues/125. If you want to bring up this use case there we can talk through some of the details.