Deploy a specific subfolder with cloud-run github action when working with a monorepo

Question

I am working with a mono repo and I would like to deploy my back-end (only) with the Cloud Run service (using the "official" google cloud run action). For this action, I need a Dockerfile at the root of my project for building my image. But my Dockerfile is under ./server.

Is there a way to use something like working-directory for the run steps ? I didn't find anything from the doc.

Or do I have to have a Dockerfile at the root of my project and the Dockerfile points to the ./server?

My project tree :

./
 .github/
   worflows/
     deploy-back.yml
  client/
    ...
  server/
    ...
    Dockerfile

# deploy-back.yml
name: Deploy to production

# ...

jobs:
  deployment-job:
    runs-on: ubuntu-latest
    environment: production
    steps:
      - uses: actions/checkout@v2
      - name: Authenticate to Google Cloud
        uses: google-github-actions/auth@v0
        with:
          # ...
      - name: Deploy to Cloud Run
        uses: google-github-actions/deploy-cloudrun@v0
        with:
          service: ${{ env.SERVICE_NAME }}
          source: gcr.io/${{ env.PROJECT_ID }}/${{ env.SERVICE_NAME }}:${{ env.PACKAGE_VERSION }}
          project_id: ${{ env.PROJECT_ID }}
          region: ${{ env.RUN_REGION }}
          tag: ${{ env.PACKAGE_VERSION }}