gitlab: convert jobs to use custom built containers

gitlab: convert jobs to use custom built containers

Now that we're building standard container images from
dockerfiles in tests/docker/dockerfiles, we can convert
the build jobs to use them. The key benefit of this is
that a contributor can now more easily replicate the CI
environment on their local machine. The container images
are cached too, so we are not spending time waiting for
the apt-get/dnf package installs to complete.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20200622153318.751107-4-berrange@redhat.com>
[AJB: tweak naming convention]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20200701135652.1366-23-alex.bennee@linaro.org>