Remove opam-repository from the dockerfiles created by dockerfile-opam

[kit-ty-kate]

See ocurrent/docker-base-images#99

To me having opam-repository is a straight downside in every regard. For the few specialized CI systems that require it for some reason they can clone it by hand and rely on the docker cache to offset the time cost.

This is a breaking change and should be announced on Discuss before merging of course. As well as carefully removing uses of opam-repository in the CI systems and Dockerfiles that rely on it.

TODO:

• opam-health-check does not rely on it (always starts with rm -rf opam-repository)
• opam-repo-ci does not use it, however its deployment dockerfiles do (fixed by Make the deployment Dockerfile resistant against changes in the base images opam-repo-ci#151)
• ocaml-ci relies on it, as well as its deployment dockerfiles
• ocluster uses a hash for both of its deployment dockerfiles
• ocaml-docs-ci seems to be clear from it in both its dockerfile and internal actions (uses opam repo set-url default ...)
• ocaml-multicore-ci uses it for both its dockerfiles and internal actions
• ocurrent uses it for its dockerfile
• mirage-ci seems to use it for at least its deployment dockerfile but I’m not sure about its internal actions
• ocurrent-deployer’s deployment dockerfile uses a hash for its base image
• current-bench: no idea
• ...

[avsm]

I still can't form an opinion about this PR without knowing which users it will break (outside of the CI systems above). Perhaps posting on discuss.ocaml.org about the proposed change would be worthwhile first, @kit-ty-kate ?

[benmandrew]

@kit-ty-kate any updates on this?