I think this is mostly because that's out of scope of responsibility of docker, and docker compose (for the most part) is only a local dev tool without prod concerns.
For deploying docker containers to production, and how to manage secrets, you'd need to look to that container orchestrator's recommendations. EG K8S secrets. It doesn't make too much sense to put an example of how to use production secrets in a docker guide, because those belong in a K8S/GKS/EKS/DO etc tutorial.
Docker's "interface" is how to accept env variables, it's other parts of the system that need to set those variables.
