Support replication for Docker container registries

Customer is trying to set up Geo and plans on using a read-only Docker registry. We should investigate the best path for replicating images across nodes.

Is the simplest method to configure the secondary as a pull-through cache? https://docs.docker.com/registry/recipes/mirror/ The downside to this is that images will only be fetched when they are needed and won't show up in the GitLab UI.

/cc: @jimash, @ayufan

Docker Distribution doesn't support proxy cache for private registries.

The method for us to have registry replication are:

1. Send events from registry to external daemon that will do docker pull source/docker push target,
2. Do rsync or s3sync over remotes.

There is interest in this from our customer base: https://na34.salesforce.com/0066100000I5nO9.

Customer mentioned they are looking to use Ceph for an on-premise S3 solution, and that they will likely use the federated replication that Ceph provides. We may still need to synchronize metadata.

added feature proposal label

changed milestone to %Next 2-3 months