Host's environment will override values in `.env`

[treygriffith]

As an example, if the user has DATA_DIR set on the host machine and then tries to run docker-compose up -d on the broker, the DATA_DIR from the host machine will take precedence over any values in the .env file.

This is likely due to - DATA_DIR=${DATA_DIR:-} which is a tactic we use to suppress warnings from docker-compose (with docker-compose saying that it is setting environment variable to an empty string).

We should find ways to avoid the hosts environment from overriding these values, or at least mitigate it through namespacing.