bash completion for docker {run,create} --userns

Signed-off-by: Harald Albers <github@albersweb.de>
(cherry picked from commit 62475684b9)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

contrib/completion/bash/docker

@@ -1668,6 +1668,7 @@ _docker_run() {
 		--tmpfs
 		--ulimit
 		--user -u
+		--userns
 		--uts
 		--volume-driver
 		--volumes-from
@@ -1827,6 +1828,10 @@ _docker_run() {
 			__docker_complete_user_group
 			return
 			;;
+		--userns)
+			COMPREPLY=( $( compgen -W "host" -- "$cur" ) )
+			return
+			;;
 		--volume-driver)
 			__docker_complete_plugins Volume
 			return