LISTMONK_db__host=localhost LISTMONK_db__user=listmonk LISTMONK_db__password=listmonk LISTMONK_db__database=listmonk listmonk --config ''

Every config.toml key maps to an env var with the LISTMONK_ prefix and double underscores for dots, so db.host becomes LISTMONK_db__host. Passing --config '' disables file config and makes environment variables the only source. This is exactly how the official docker-compose.yml configures the container.