Don't recommend modifying docker-compose file to set sub-URI

[fbacall]

In https://docs.seek4science.org/tech/docker/docker-compose.html#using-a-sub-uri it mentions editing docker-compose.yml to set RAILS_RELATIVE_URL_ROOT.

This could cause conflicts etc. when upgrading, so it would be better to keep such changes in another .env file that is not under version control.

It is possible to reference multiple env_files in docker-compose, so we could have something like:

    env_file:
      - docker/db.env
      - .env

and recommend the user creates .env and puts any custom variables in there.