handle DOKKU_TRACE

Author: Cactusbone

pre-build-buildstep

@@ -1,4 +1,7 @@
-#!/bin/bash -e
+#!/usr/bin/env bash
+
+set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+
 APP="$1"; IMAGE="dokku/$APP"
 echo "-----> Injecting apt repositories and packages ..."