web: Don't ignore changes in meta data of containers and images.

The meta data of containers and images was only recorded the first
time we saw a given id, and never updated.

One symptom was that changes to the RepoTags of an image would not be
noticed, and new images that start out as "<none>:<none>" would never
be listed even if they later get a name, such as with "docker commit".

Closes #404

Reviewed-by: Stef Walter <stefw@redhat.com>