ci: Move image tags into a separate template yml

ci: Move image tags into a separate template yml

This will allow gst-plugins-rs documentation CI to include this
template via a remote include and doesn't need to manually update the
image. This causes cascading failures across all repos, for example
https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/874
is required for making gst-plugins-rs CI pass in Cerbero, which is
also run on the gstreamer monorepo.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3065>