gatsby build and deploy to firebase

wercker.yml

box: node
build:
  steps:
    - script:
      name: npm build
      code: |
        npm install -g gatsby
        yarn install
        gatsby build
deploy:
  steps:
    - script:
      name: install firebase-tools
      code: |
        npm install -g firebase-tools
    - script:
      name: deploy firebase
      code: |
        firebase deploy --project "$FIREBASE_PROJECT" --token "$FIREBASE_TOKEN" --only hosting