fix(registry): bind to $HOST if present

docker-registry by default binds itself to 0.0.0.0. When we are in a
public cloud which exposes all ports by default, this means that the
registry will accept all incoming connections from anyone. This is not
intentional. The registry should only accept connections internal to
the cluster. Setting REGISTRY_HOST binds the registry to that address.

        registry@8fb662d4e86a:/app$ docker-registry --help
        usage: docker-registry [-h]

        run the docker-registry with gunicorn, honoring the following
        environment variables:
        REGISTRY_HOST: TCP host or ip to bind to; default is 0.0.0.0

fixes #2479