ci: Start using FDO_REPO_SUFFIX for clearer image names (GNOME 46)
Currently, containers get stored based on their Fedora version, but that
can make it hard to see which container is used in which branch.
Instead, we can use FDO_REPO_SUFFIX
to sort the containers based on
their branch name.
This would also make it easier to see which containers can be removed, as there are currently gigabytes of unused containers it seems.
To make it as neat as possible, the images now get stored as
main/2024-05-19.0-fedora-rawhide
for example.
Based on !2610 (merged) for main.
See https://gitlab.gnome.org/velsinki/gnome-control-center/container_registry for how that looks.