CI: use the registry REST API
This MR should clean up a little bit the CI for libinput:
- there is no point pulling the entire docker image if we are just interested in the creation date
- we should properly delete the old images instead of just overwriting the tags
Last point is in the 'completely blurry' category, but better being safe and actually make sure the registry don't keep handles on the blobs so they will get cleaned up after 30 days.
Note: the 'new' pipeline can be seen at https://gitlab.freedesktop.org/libinput/libinput/pipelines/1451