Allow for nginx proxy by subfolder

[fabriziorizzo]

provide a way to override/customize the base URL so openaudible can be hosted behind a centralized nginx proxy in the fashion of linuxserver.io's swag container:
see: https://github.com/linuxserver/docker-swag

[openaudible]

This seems like a job for your reverse proxy. I use Traefik and that works.

Feel free to propose a solution.