Improve container actions

[jmgilman]

Actual production use of containers typically involves doing the following:

1. Building the image
2. Publishing the image with multiple tags to multiple registries

Currently, we can accomplish the first one. The second one requires adding support for the following:

1. Introducing an attribute to allow specifying multiple registries
2. Introducing an attribute to allow specifying multiple tags
3. When the publish action is called, multiplexing the above to create n number of calls to upstream

There are upstream issues for accomplishing these, but progress is slow, and the maintainer doesn't seem interested in supporting them.

[blaggacao]

I think this is now actually implemented at least for tags?

[blaggacao]

from #318

  # Currently, there is no good documented way to publish to arbitrary repositories at once
  # NOTE: untested
  p2n-env-gl = cell.oci.p2n-env-oci.overrideAttrs {
    meta.repo = "gitlab.<domain>.com/<group>/<repo>/<image>";
  };