ci: Update container image URL

The container image used for CI is now built from a project in the
gitlab.gnome.org/GNOME/ namespace, rather than my personal namespace.
1 job from sam/ci-image-url in 2 minutes and 14 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #182592
test-fedora-latest

00:02:14