HTTP 404 Error setting hooking up Google’s Cloud Run to Firebase Hosting

Solution
"hosting": {
  // ...

  // Add the "rewrites" attribute within "hosting"
  "rewrites": [{
    "source": "/helloworld",
    "run": {
      "serviceId": "helloworld",  // "service name" (from when you deployed the container image)
      "region": "us-central1"     // optional (if omitted, default is us-central1)
    }
  }]
}