.gitlab-ci.yml: adapt the docker build jobs to be more like citemplates

.gitlab-ci.yml: adapt the docker build jobs to be more like citemplates

This is the inital step towards migrating our docker images setup
to something closer and eventually freedesktop/citemplates [1]

The idea is that jobs always run from the registry in your fork. If the
image sha/id matches the one from the upstream registry, its copied
over else a new one is build, pushed and tested.

Only change the fedora job for now while testing.

[1]: https://gitlab.freedesktop.org/freedesktop/ci-templates

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-ci/-/merge_requests/308>