buildah test helper: run registry:2.6, not :2
    
    The docker.io/registry:2 image broke suddenly and with no warning:
    
    https://github.com/docker/distribution-library-image/issues/106
    
    Symptom: running 'htpasswd' from that container no longer works,
    even though it's been documented for years (at docker.com) and
    as of this writing still remains documented.
    
    One solution is to run htpasswd locally (requires installing the
    httpd-tools package) or using any crypt library to hash. Not
    hard, but tedious, and it doesn't address the issue of the
    docker registry:2 image being a moving target.
    
    I choose to force the use of the :2.6 tag, in hopes that that
    image will remain unmodified. Refactored to use FQIN instead
    of shortname, and to define it in only one place.
    
    Signed-off-by: Ed Santiago <santiago@redhat.com>