ci: create micro-image for cerbero trigger jobs
The fdo placeholder job runners often re-pull a gstreamer image that they really should've cached already, for unknown reasons (bug somewhere possibly).
This delays the cerbero trigger by multiple minutes (3-5 or so), which in turn delays the start of the cerbero sub pipeline by that much time then, which in turn delays completion of the MR pipeline, because the cerbero sub-pipeline tends to be what takes the longest overall.
We could just make a micro image for the cerbero trigger job. Probably doesn't need much but python-gitlab installed. Now it pulls the full fedora image which is quite large.
Also applies to gst-plugins-rs