fix: skip clearing cache if we are not building a herokuish image

Closes #2771

plugins/apps/pre-delete

@@ -8,6 +8,10 @@ apps_pre_delete() {
   local APP="$1"; local IMAGE_TAG="$2"; local IMAGE=$(get_deploying_app_image_name "$APP" "$IMAGE_TAG"); local CACHE_DIR="$DOKKU_ROOT/$APP/cache"
   verify_app_name "$APP"
 
+  if ! is_image_herokuish_based "$IMAGE"; then
+    return
+  fi
+
   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
   fi