Try generating the requirements.txt in the Dockerfile

If we installed a new package (or we updated something) we need to run 

```sh
pipenv lock -r > requirements.txt
```

which will generate https://github.com/mattborghi/background-tasks-queue/blob/main/backend/requirements.txt

That's not the best idea. I think heroku detects the Python environment because of the Pipfiles, but we have to test it.