Auth with Cloud Run Service Account? #3623

I can do local auth with the below code using my service account credentials. Set in .env as GOOGLE_APPLICATION_CREDENTIALS=./service-account-key.json

try {
    const auth = new google.auth.GoogleAuth({
      scopes: SCOPES,
      clientOptions: {
        subject: "user@email.com",
      },
    });
    const gmail = google.gmail({ version: "v1", auth });
    return gmail;
  } catch (e) {
    console.error(e);
    return null;
  }

But I'm trying to figure out how to get this working on Cloud Run using the service account on there.. I assumed the auth stuff would be passed in automatically, which doesn't seem right, but also couldn't find any documentation to show otherwise. Has anyone done this before? Any alternatives I can try here?