Add support for IPv6 addresses in --dns parameters.

Docker-DCO-1.1-Signed-off-by: Jan Pazdziora <jpazdziora@redhat.com> (github: adelton)
add check on docker run

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

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
Merge pull request #6920 from vieux/pr_6408

Add support for IPv6 addresses in --dns parameters.