[Docker] retiring old distributions

[avsm]

Right now older distributions such as Ubuntu 15.10 are simply not rebuilt, but the relevant tags are not deprecated. The effect is that it is invisibly possible to depend on an old OPAM repository.

I'm not sure what the right thing to do here.

• Delete the tag: this will break any repositories that depend on that for CI.
• Leave as-is: security holes mount up in the base distro and OPAM repo is outdated. Leads to issues like this one in MirageOS.
• Rebuild one last time with a big warning: so that CI systems at least see the problem.

If we do the latter, then we can also document prominently the right aliases to use (e.g. ocaml/opam:alpine instead of ocaml/opam:alpine-3.4) so that the latest version is retrieved rather than pinning to something that will be difficult to rebuild in the future.

[dra27]

Cross-referencing https://github.com/ocurrent/docker-base-images/issues/57.