Add support for DOKKU_SCALE

[john-doherty]

It would be great if you could add support for creating/overwriting the DOKKU_SCALE file

    - uses: idoberko2/dokku-deploy-github-action@v1
      with:
          ssh-private-key: ${{ secrets.DOKKU_SSH }}
          dokku-host: 'myapi.com'
          app-name: 'api'
          dokku-scale: 'web=2'

Could execute

echo web=2 > DOKKU_SCALE