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>

support/docker/Dockerfile

@@ -1,7 +1,7 @@
 # This Dockerfile generates the docker image that gets used by Gitlab CI
 # To build it (YYYYMMDD.HHMM is the current date and time in UTC):
-#   docker build -t buildroot/base:YYYYMMDD.HHMM support/docker
-#   docker push buildroot/base:YYYYMMDD.HHMM
+#   docker build -t registry.gitlab.com/buildroot.org/buildroot/base:YYYYMMDD.HHMM support/docker
+#   docker push registry.gitlab.com/buildroot.org/buildroot/base:YYYYMMDD.HHMM
 
 # We use a specific tag for the base image *and* the corresponding date
 # for the repository., so do not forget to update the apt-sources.list