Dockerfile templating should be removed from ImageBuilder

[lbussell]

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

• dotnet/dotnet-docker
• 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 and consolidation of NetFX 4 images. 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:

• Migrate off of Cottle templating engine #1610

[lbussell]

[Triage] Buildtools-prereqs has an open issue for using Dockerfile templating:

• Implement solution for sharing common content used in Dockerfiles dotnet-buildtools-prereqs-docker#1393

From discussion, there's desire to keep any Dockerfile templating infrastructure in this repo, just not necessarily coupled directly to ImageBuilder. The team is on-board with simplifying/speeding up templating.