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

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

Start with the referenced src/aspnet/8.0/alpine3.18/arm32v7/Dockerfile and the Docker buildx build-context documentation. Trace how this repository generates or builds Dockerfiles, then identify the affected parameterized FROM statements and build commands. Done means the relevant Dockerfiles use build contexts for dynamic base-image replacement without the current source-repository parameterization.

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