Deploy registry mirror instance for DockerHub and deprecate Dependency Proxy

After experimenting with the dependency proxy, we found it wasn't sufficient to completely fix CI regularly hitting DockerHub rate limits. In particular, contributors without permissions in the context of the parent projects cause the runners to continue to pull directly from docker.io and spend our precious pulls.

As a replacement, we'll deploy a container registry configured as a pull-through cache for docker.io and configure our GitLab runners to pull DockerHub images through this mirror. The firewall will be set up to block all pulls except those from our own runners. This will be quite similar to how the OSUOSL have configured their own runners.