Delete the /cache directory's contents from inside the container.

Fixes #322.
Buildpacks write into /cache as root, which made the delete command unable
to delete it.
Its contents are now deleted using "docker run", and thus as root.

plugins/00_dokku-standard/pre-delete

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+
+APP="$1"
+IMAGE=app/"$1"
+CACHE_DIR="$HOME/$APP/cache"
+
+docker run -v $CACHE_DIR:/cache $IMAGE find /cache -depth -mindepth 1 -maxdepth 1 -exec rm -Rf {} \;
+
+