Add overlays repo on Windows

[dra27]

~This is WIP - opening with a trial version to test the fix.~

This applies a previous version of the change in https://github.com/ocurrent/docker-base-images/pull/147 to Dockerfile_opam.all_ocaml_compilers and Dockerfile_opam.separate_ocaml_compilers, but these two functions aren't used anywhere anymore.

[dra27]

@avsm - I think these two functions were for obi and aren't being used/tested anywhere. Up to you whether it's worth putting this in, therefore (I only landed on updating them because the Windows changes had previously been made by @MisterDA!)

[MisterDA]

This is becoming interesting again, also because opam-repository-mingw is starting to be too old. It's odd that this PR wasn't merged sooner. I'm tempted to test this and add the ocaml/opam-repository to Windows images too.

[dra27]

This didn't need to be merged for the base images - the repo is added there