Drop custom wrapper around Dockerfile

[nbars]

Instead of generating a Dockerfile based on the content of settings.yml, allow exercises to be defined as Dockerfiles. This allows much shorter round-trip times for testing and can also be used to provide the exercise as an image for local testing.

[nbars]

One disadvantage is that this does not integrate well with exercises that spawn multiple containers per user (e.g, a service the user is supposed to attack from its primary container).