allow tagging and deployment of tagged app images

plugins/tag/functions

@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+source "$(dirname $0)/../common/functions"
+
+tag_image() {
+  docker tag -f "$1" "$2"
+}