I've rebuilt the new Docker image we use for CI to include GStreamer in
the dependencies.
We really need to have the Docker registry hosted on gnome.org, to avoid
pointing people at Dockerhub.
The CI runner is pretty slow to set up (takes about 6 minutes to get
through the system dependencies needed to build GTK), and does not work
with dependencies as subprojects.
Until we figure out how to make it work, and make it work a bit faster,
we should drop CI and rely on Continuous for a while longer.
We can revert this commit as soon as we find out how to make things
work.
