Use .env in docker compose config

[Pwuts]

All the configuration for the docker setup is currently hardcoded in docker-compose.platform.yml. I think it's a better idea to load these values from .env instead.

This means creating autogpt_platform/.env.example from a fusion of supabase/docker/.env.example and backend/.env.example (and frontend/.env.example?)

Before making a huge .env.example, it's probably useful to also remove the Supabase services that we don't use from docker-compose.yml.