Use Docker buildx build context to dynamically replace FROM statements in Dockerfiles

[lbussell]

Docker buildx has a feature that allows you to dynamically override any of the FROM statements in a Dockerfile at build-time. For example, --build-context mcr.microsoft.com/dotnet/runtime:8.0=mycustomacr.azurecr.io/repo:tag.

https://docs.docker.com/reference/cli/docker/buildx/build/#build-context

This means we wouldn't need to parameterize the source REPO of many of our Dockerfiles like we do today, making our Dockerfiles cleaner:
https://github.com/dotnet/dotnet-docker/blob/87b3735c6c2ba37277be80dd04099f6cb93e4bb7/src/aspnet/8.0/alpine3.18/arm32v7/Dockerfile#L1-L2

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[MichaelSimons]

One question is if we would do this for linux only as Windows doesn't have official support for buildx yet (currently in-progress)?

[mthalman]

This would allow us to get rid of this functionality too: #1031

[lbussell]

[Triage] This would be nice to have. We need to do more investigation on the Windows BuildKit support (but that shouldn't necessarily block Linux adoption).