Fix the custom builder unit test

It was broken since ssh support was introduced.
Compatibility with Docker 0.4.1 (closes #23)

- The regressions introduced in Docker 0.3.3 have been addressed in
  Docker 0.4.1;
- Sandbox is now compatible with the new docker inspect format (it
  returns a list now );
- Do not use cat /dev/zero in an unit test when the output is read by
  Docker!