now.json

{
  "build": {
    "env": {
      "FIREBASE_API_KEY": "@firebase-api-key",
      "FIREBASE_AUTH_DOMAIN": "@firebase-auth-domain",
      "FIREBASE_DATABASE_URL": "@firebase-database-url",
      "FIREBASE_PROJECT_ID": "@firebase-project-id",
      "FIREBASE_STORAGE_BUCKET": "@firebase-storage-bucket",
      "FIREBASE_MESSAGING_SENDER_ID": "@firebase-messaging-sender-id",
      "FIREBASE_APP_ID": "@firebase-app-id"
    }
  }
}