Trick Github Actions: Pull docker image from private registry

github-actions-trick.yml

- uses: docker://[ref-to-public-image]
- name: "pull private image and retag it" 
      env: 
        DOCKER_USERNAME: ${{ secrets.DOCKER_USERNAME }}
        DOCKER_PASSWORD: ${{ secrets.DOCKER_PASSWORD }}
        DOCKER_REGISTRY_URL: ${{ secrets.DOCKER_REGISTRY_URL }}
      run: | 
          docker login -u "$DOCKER_USERNAME" -p "$DOCKER_PASSWORD" ${{ secrets.DOCKER_REGISTRY_URL }}
          docker pull [url-of-private-image]
          docker tag [url-of-private-image] [ref-to-public-image]
- name: 'this step will be using the private image'
      uses: docker://[ref-to-public-image]
      with: 
        entrypoint: /bin/bash
        args: random-script.sh