Allow --net=none & -h

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
add tests

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
Merge pull request #6365 from vieux/allow_net_none_h

Allow --net=none and -h