How does the Docker toolchain environment file work?

The Docker toolchain menu has an option for specifying an environment file, but I don't see any documentation for what this file is supposed to be? Is this a shell script that runs inside the Docker container? Is this just a .env file whose values are set as environment variables in the Docker container? I've tried both approaches, but this file seems to have no effect on the running containers.

https://www.jetbrains.com/help/clion/clion-toolchains-in-docker.html#create-docker-toolchain