By proxy by vendoring containers/common. Previously, a "dangling" image
was an untagged image; just a described in the Docker docs. The
definition of dangling has now been refined to an untagged image without
children to be compatible with Docker.
Further update a redundant image-prune test.
Fixes: #10998Fixes: #10832
Signed-off-by: Valentin Rothberg <rothberg@redhat.com>