A .env file allows you to go from knowing/assuming where a particular file is, to having environment variables.
What I'm describing is that you can go from being passed environment variables directly, to knowing where certain files are; which therefore allows those files to be mounted into a container-image in arbitrary places, rather than encoding brittle assumptions about config-file locations into the binary within the container-image — assumptions that usually get in the way during development, when you're not running the binary inside a container.
You still didn't understand my point. The thing that I'm talking about is using env-vars to point at config files that do not themselves contain env-vars, but rather are opaque app-specific config files, in formats like YAML or whatever-else.