Enable tracing in hooks when DOKKU_TRACE is set.

This makes it much easier to debug plugins.
Also source the 'dokkurc' file when starting dokku. This makes it easy
to define DOKKU_TRACE over ssh.

plugins/git/commands

@@ -1,5 +1,5 @@
 #!/usr/bin/env bash
-set -eo pipefail
+set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
 
 case "$1" in
   git-hook)