DOC: Architecture - Required env vars and split into containers

[ghsa-retrieval]

What type of documentation would you like?
Documentation about the architecture of DejaCode and how its components interact with each other and other systems (postgres, redis, scancode, ldap, ...) for authentication

Documentation topic
I'm currently attempting to build a helm chart for DejaCode based on the provided Docker Compose solution, but the current architecture left some questions not answered by current documentation. I have asked about this on gitter and Philippe Ombredanne instructed me to file a ticket.

1. Recently the scheduler has been introduced for updating vulnerability information. In Docker Compose it is run as a separate container. Is there any reason it is not simply a background task in the worker?
2. Currently all containers started by compose share environment variables. Most likely web, worker and scheduler do not all need access to the DB and redis password as well as other env vars. Which ones are actually needed for which container? Relevant env vars are:
   • DATABASE_PASSWORD
   • REDIS_URL
   • DEJACODE_REDIS_PASSWORD
   • SECRET_KEY
   • SECRET_KEY_FALLBACKS
   • EMAIL_HOST_PASSWORD
   • AUTH_LDAP_BIND_PASSWORD
   • SCANCODEIO_API_KEY
   • PURLDB_API_KEY
   • VULNERABLECODE_API_KEY