support/docker: drop use of Docker hub, use Gitlab registry

Now that our pipelines are using the Docker image from the Gitlab
registry, there is no longer any reason to push the image to the
Docker hub.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Romain Naour <romain.naour@gmail.com>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>