Add a new artifact type: container-image
    
    Following up <https://pagure.io/fedora-ci/messages/issue/17>
    
    Add a new artifact type `container-image` for representing a generic
    container image built by CI/CD pipelines or anywhere outside of OSBS.
    
    A `container-image` is identified by a repo name and a digest (usually
    a sha256 digest).
    
    For now, only `test.complete` and `test.error` messages are defined.