Add zsh completion for 'docker service create --hostname'

Signed-off-by: Steve Durrheimer <s.durrheimer@gmail.com>

contrib/completion/zsh/_docker

@@ -1672,6 +1672,7 @@ __docker_service_subcommand() {
                 $opts_create_update \
                 "($help)*--container-label=[Container labels]:label: " \
                 "($help)*--env-file=[Read environment variables from a file]:environment file:_files" \
+                "($help)--hostname=[Service containers hostname]:hostname: " \
                 "($help)--mode=[Service Mode]:mode:(global replicated)" \
                 "($help)--name=[Service name]:name: " \
                 "($help -): :__docker_complete_images" \