Remove test from built target

This removes the testing from the build targets. I think it should be two distinct steps - someone just wanting to build the docker image should not have to go the (rather lengthy) test procedure. This MR fixes that - testing must now be run explicitly.

Also fixed problems around ssh-agent influencing our tests by completely isolating the test (avoiding the use of ssh-agent).

Added some documentation around the make targets we have.

Closes #29 (closed).