Refactor the docker image to correspond to the minimum necessary for user execution

[jamesemery]

Right now our docker image is much larger than it needs to be. This is at least in part because it contains our entire git clone as well as the packaged jars. This is not necessary and could potentially come with it shrinking our docker image substantially.

This change would involve a major refactoring of how we execute our tests through gralde inside the docker image, as removing the test dependencies will mean we probably have to externally mount the git clone from the docker image in order to pull in the proper dependencies.