Testbed - deprecate use of the ENV file, and instead use the environment parameter of container.run()

[afourney]

In the Testbed, environment variables are handled by populating and ENV file and sharing it with the Docker container via a shared folder. However, we then need to be very sure to delete this file when sharing the results of a run. The current script handles deletion in most cases, but not if Docker times out, or if there is a keyboard interrupt. By passing environment variables through the sanctioned Docker mechanism, we remove the need to manage this file.

[afourney]

Completed in #1048