Deploy Firebase functions with Github Actions

deploy-functions.yml

name: Build and Deploy
on:
  push:
    branches:
      - main

jobs:
  deploy:
    name: Deploy
    runs-on: ubuntu-latest
    steps:
      - name: Checkout Repo
        uses: actions/checkout@v2.3.4
      - name: NPM install with caching (functions)
        uses: bahmutov/npm-install@v1.6.0
        with:
          working-directory: functions
      - name: Deploy to Firebase
        uses: w9jds/firebase-action@v2.0.0
        with:
          args: deploy --only functions
        env:
          FIREBASE_TOKEN: ${{ secrets.FIREBASE_TOKEN }}

Get the Firebase token with firebase login:ci

Read here how to set this token on Github.