Add bash completion for `docker service create --hostname`

Signed-off-by: Harald Albers <github@albersweb.de>

contrib/completion/bash/docker

@@ -2621,6 +2621,7 @@ _docker_service_update() {
 			--container-label
 			--env-file
 			--group
+			--hostname
 			--mode
 			--name
 		"