Add x-maintenance-intent to your opam files

[hannesm]

Dear Madam or Sir,

due to the opam repository archival process, we are keen to have the maintainers of packages specify their intention. We noticed that dockerfile has a lot of releases, and are curious what your maintenance intention is.

You can read up on the policy at https://github.com/ocaml/opam-repository/blob/master/governance/policies/archiving.md and the x-maintenance-intent field at https://github.com/ocaml/opam-repository/blob/master/governance/policies/archiving.md#specification-of-the-x--fields-used-in-the-archiving-process

Thanks a lot for taking this into consideration. Please note that adding x-maintenance-intent in the latest release is sufficient (please feel free to PR such a field directly to the opam-repository).

Also note that if there exists a package that is to be retained in opam-repository, and this depends on a specific dockerfile version (not part of the maintenance-intent), this dockerfile version is kept -- so we don't end up in uninstallable packages.

Please note that the same opam field should be added to other ocurrent projects. Thanks in advance.