fix: respect DOKKU_APP_USER in is_image_herokuish_based

plugins/config/docker-args-deploy

@@ -14,7 +14,7 @@ config_docker_args() {
   trigger="$0 config_docker_args"
   verify_app_name "$APP"
 
-  if ! is_image_herokuish_based "$IMAGE"; then
+  if ! is_image_herokuish_based "$IMAGE" "$APP"; then
     ENV_ARGS="$(config_export app "$APP" --format docker-args --merged)"
     echo -n "$STDIN $ENV_ARGS"
   else