ci: Fetch list of available container images dynamically

ci: Fetch list of available container images dynamically

Any static list of images is destined to become outdated eventually,
so let's start generating it dynamically instead.

Unfortunately there doesn't seem to be a straightforward way to get
Podman/Docker to list all repositories under quay.io/libvirt, so we
have to resort to searching and filtering manually; and since the
two tools behave slightly differently in that regard, it's more
sane to have the logic in a separate shell script than it would be
to keep it inline in the Makefile with all the annoying escaping
doing so would entail.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Cole Robinson <crobinso@redhat.com>