Thanks to @MasterofJOKers, here is basic support for SSL:
You need to create your key and certificate and copy them to the `data`
directory, named `ssl.cert` and `ssl.key`.
Then, set `PAPERLESS_USE_SSL` in `docker-compose.env` to `true`.
Solves #576.
This file, like `docker-compose.yml`, should be edited by the user. To
avoid merge conflicts when pulling updates, the edited version should
not be committed to the repository.
