Pull the deploying app image name where necessary

All of these commands should execute against the image that is in use versus the "latest" that dokku provides the system with.

plugins/repo/subcommands/purge-cache

@@ -9,7 +9,7 @@ repo_purge_cache() {
   local APP="$2";
   verify_app_name "$APP"
 
-  local IMAGE=$(get_app_image_name "$APP"); local CACHE_DIR="$DOKKU_ROOT/$APP/cache"
+  local IMAGE=$(get_deploying_app_image_name "$APP"); local CACHE_DIR="$DOKKU_ROOT/$APP/cache"
 
   if [[ -d $CACHE_DIR ]]; then
     docker run "$DOKKU_GLOBAL_RUN_ARGS" --rm -v "$CACHE_DIR:/cache" "$IMAGE" find /cache -depth -mindepth 1 -maxdepth 1 -exec rm -Rf {} \; || true