Dockerfile templating should be removed from ImageBuilder

Of all the repos that use ImageBuilder, only two use its templating features:

- [dotnet/dotnet-docker](https://github.com/dotnet/dotnet-docker)
- [microsoft/dotnet-framework-docker](https://github.com/microsoft/dotnet-framework-docker)

dotnet-docker makes heavy use of templating to generate hundreds of Dockerfiles.

dotnet-framework-docker, on the other hand, is comparatively simple. And in the coming months it will get even simpler, thanks to the [removal of NetFX 3.5 images](https://github.com/microsoft/dotnet-framework-docker/discussions/1249) and consolidation of [NetFX 4 images](https://github.com/microsoft/dotnet-framework-docker/discussions/1249). In fact, after that consolidation it will have _only 16_ Dockerfiles managed by templates.

My argument is that those two repos would be better served by their own solutions which are closer in proximity to their own usage. And neither solution needs to be coupled to a full containerized build environment. Being too tightly coupled to ImageBuilder really slows down the dev loop for writing Dockerfiles.

Related:
- https://github.com/dotnet/docker-tools/issues/1610