Add bash completion for `system df --format`

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

contrib/completion/bash/docker

@@ -4173,9 +4173,15 @@ _docker_system() {
 }
 
 _docker_system_df() {
+	case "$prev" in
+		--format)
+			return
+			;;
+	esac
+
 	case "$cur" in
 		-*)
-			COMPREPLY=( $( compgen -W "--help --verbose -v" -- "$cur" ) )
+			COMPREPLY=( $( compgen -W "--format --help --verbose -v" -- "$cur" ) )
 			;;
 	esac
 }

docs/reference/commandline/system_df.md

@@ -21,8 +21,9 @@ Usage:	docker system df [OPTIONS]
 Show docker filesystem usage
 
 Options:
-      --help      Print usage
-  -v, --verbose   Show detailed information on space usage
+      --format string   Pretty-print images using a Go template
+      --help            Print usage
+  -v, --verbose         Show detailed information on space usage
 ```
 
 ## Description