Easily re-tag images without pulling first

Summary

With the new container registry, we hope people will use it to store intermediate images during CI. One common use-case is to test an image, and then once it passes, tag the image as latest. If this is done in a separate deploy stage, however, the runner will (usually) be missing the current image and thus have to docker pull before docker tag and finally docker push. The registry supports adding a tag directly to an already stored image, but the docker CLI does not support this. We should somehow make this available to CI users so they can re-tag an image without having to pull first.

Options

There are a few different ways we could do this:

• Add to GitLab CLI (but since we don't offer one ourselves, this implies adopting some third-party CLI)
• Patch Docker CLI and send upstream merge request
• Document a curl script to use existing registry API
• Add a plugin that does it.