on projectatomic-based docker, we get "docker.io/library/busybox:latest"
when someone uses an unqualified name like "busybox". Though when we
inspect, the RepoTag will still say "docker.io/busybox:latest", So
we have reparse the tag, normalize it and try again. Please see the
additional test case.
Signed-off-by: Andy Goldstein <andy.goldstein@gmail.com>