Support for using dyno metadata during build (#13)

bin/compile

@@ -56,7 +56,9 @@ cp "$BP_DIR/.profile.d/inject_react_app_env.sh" "$profile_d_dir/"
 #   * https://github.com/facebookincubator/create-react-app/pull/476
 # * `NPM_*`, especially for `NPM_TOKEN`
 #   * https://docs.npmjs.com/private-modules/ci-server-config
-export_env_dir "$ENV_DIR" '^(REACT_APP_|NODE_|NPM_)'
+# * `HEROKU_*`, especially for Dyno Metadata
+#   * https://devcenter.heroku.com/articles/dyno-metadata
+export_env_dir "$ENV_DIR" '^(REACT_APP_|NODE_|NPM_|HEROKU_)'
 
 # Build the javascript bundle
 npm run build