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