Add docker ps --filter=… image completion

I missed that on #15919, thanks @albers

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

contrib/completion/bash/docker

@@ -975,6 +975,11 @@ _docker_ps() {
 	esac
 
 	case "${words[$cword-2]}$prev=" in
+		*ancestor=*)
+			cur="${cur#=}"
+			__docker_images
+			return
+			;;
 		*id=*)
 			cur="${cur#=}"
 			__docker_container_ids