gitlab-ci: use skopeo to check on container images
this allows to replace complex curl queries with simpler commands.
We need a newer minimalist image with skopeo in addition to jq and curl.
Also, I am currently not relying on skopeo to delete the image as I am not so sure we will get the same cleanup than with the current sha method and also: https://bugzilla.redhat.com/show_bug.cgi?id=1481196