Use ex_spec and alias for more readable tests.
Pull out a docker test that cannot be run during other tests.
Make timeout configurable for tests. Increase timeout since it includes the container launch time.
Extract the docker-in-docker pieces of the Dockerfile to a new image:

https://github.com/pgr0ss/elixir-docker-in-docker
Pull down the Docker image on start. Closes #1.

This prevents the first tests or usages from timing out.
Move problem_test.exs into models directory to match code structure.