Remove detection logic for docker environment

[zachmullen]

In order to get DJDT to work in our docker-compose configuration, we had to add some conditional logic specific to the docker environment, leading to the creation of the _is_docker function.

In lieu of this function, we could read an environment variable that declares that the application is running in docker. For backward compatibility, we could fall back to the existing _is_docker implementation if it's not set.

If we decide to make this change, we should add the proposed environment variable to the docker compose and native environment files in the Resonant cookiecutter.