Add bash completion for `docker version --format`

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

contrib/completion/bash/docker

@@ -3690,9 +3690,15 @@ _docker_top() {
 }
 
 _docker_version() {
+	case "$prev" in
+		--format|-f)
+			return
+			;;
+	esac
+
 	case "$cur" in
 		-*)
-			COMPREPLY=( $( compgen -W "--help" -- "$cur" ) )
+			COMPREPLY=( $( compgen -W "--format -f --help" -- "$cur" ) )
 			;;
 	esac
 }