Optimize Dockerfile by not installing desktop

* Get a smaller docker image by not installing the whole ubuntu-desktop
* Some optimization to be able to run trunk tests with only the needed
  runtime and test dependencies.
Reshape Dockerfile to have fewer layers

* Avoid having too many layers and cleanup content. This avoid saving some
  MB in the image size
Depend on nose-json and produce json output

* We now rely on nose-json to publish json test results. That will enable
  easier parsing in the future that just using the raw xml.
Merge remote-tracking branch 'origin/master'