docker-registry/Dockerfile.template: Adjust Python 2 dependency

docker-registry/Dockerfile.template: Adjust Python 2 dependency

Now that the ancestor gentoo-python image is removing Python 2, we
need to get it back for docker-registry.  We need to keep the
USE_PYTHON setting (with a tweaked value) because Gunicorn and
Python-RSA haven't transitioned to the *-r1 Python eclasses yet:

  docker run --rm wking/docker-registry /bin/bash -c 'grep inherit /var/db/pkg/  */*/*ebuild' | grep distutils | grep -v -- '-r1'
  /var/db/pkg/dev-python/rsa-3.1.1/rsa-3.1.1.ebuild:inherit distutils
  /var/db/pkg/www-servers/gunicorn-0.17.4/gunicorn-0.17.4.ebuild:inherit distutils eutils