Allow configuration overrides

[PGijsbers]

This PR adds documentation (and the possibility) to allow overrides for the src/config.toml configuration and the .env configuration. This is useful, because then you can customize your own deployment (also production), without affecting files which are checked into git, which means:

• significantly less risk of accidentally checking in your custom configuration (potentially leaking secrets)
• easier to pull in updates, as previously changed configurations would lead to merge conflicts or otherwise needed to be undone before a pull

I also removed the passing around of environment variables between all docker images, as the relevant variables were already also passed through the environment field. Now containers are only aware of the variables they need.

How to Test

Change some configurations in src/config.override.toml and override.env as described in the documentation with some custom options (whatever you are already using, or something new just to test). up the containers, poke around, down the containers.

[AdrianAlcolea]

As we are already talking about that I close the revision here and we can start another one ;p

[PGijsbers]

I also had the PYTHON_PATH... I think we should probably just bake that into the docker image, though? Since it's required for the image to function even if no code is mounted... otherwise, things seem to work. All connectors are able to import data now and I can search both directly with the API and through the elastic search endpoints.

[AdrianAlcolea]

Done, everything works for me now =)

[Taniya-Das]

works well for me now.