The Docker API seems to respond with a 'null' value for the 'Labels'
attribute from containers that were created with older Docker versions.
An empty dictionary is returned in this case.
Signed-off-by: Frank Sachsenheim <funkyfuture@riseup.net>
Seems like this is pretty much ignored by Docker, so it wasn't
causing any visible issues, except when a volume name was used
instead of a path.
Also, added integration tests.
Ref #1380
Signed-off-by: Ben Firshman <ben@firshman.co.uk>