As it's currently impossible for us to create new automated
builds on Docker Hub (see [1]), and quay.io doesn't suffer
from the same problem while still having all the feature we
need, switch to the latter.

[1] https://github.com/docker/hub-feedback/issues/1676Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>