Add bash completion for docker system df

Signed-off-by: Harald Albers <github@albersweb.de>
Upstream-commit: fcf085a9f5
Component: cli

components/cli/contrib/completion/bash/docker

@@ -3117,9 +3117,12 @@ _docker_system() {
 	esac
 }
 
-# TODO new command
 _docker_system_df() {
-	:
+	case "$cur" in
+		-*)
+			COMPREPLY=( $( compgen -W "--help --verbose -v" -- "$cur" ) )
+			;;
+	esac
 }
 
 _docker_system_events() {