fix(registry): fix upstream changes

Some things changed with the version bump like how the docker registry process is
started and where dependencies are installed from.
feat(controller): add PUBLIC_INDEX_URL setting

This setting is useful for id index.docker.io ever changes.
fix(controller): import image from remote registry

This change allows us to import an image from a remote repository,
thanks to a new feature implemented in
https://github.com/deis/docker-registry/compare/repository-import.

Though the infrastructure in place allows us to import an image from the public index,
the endpoints are closed-source and therefore are not implmented in the upstream
repository-import branch.