Refactor environment variables to make them simpler

The current use of environment variables includes overwriting values (from export to docker to backend entrypoint to backend settings), using a json-like env var, reassigning variables that can be static (such as `DB_DEFAULT_ENGINE`), and using default values for variables that should be always set (such as `DB_DEFAULT_NAME`). Those are a couple of things that we could improve in our code

Trace how environment variables flow from export through Docker, the backend entrypoint, and backend settings. Inventory variables that are overwritten, JSON-like, reassigned, or given inappropriate defaults, then clarify the intended single source and required values for each. Done means the environment configuration is simpler and those redundant transformations are removed.

Written by the indexing model from the issue text.