Add proper tests for the docker image

[derschatta]

Ideally the docker images should be covered by tests so that if something breaks after a change we know immediately and can fix it before someone starts using a broken image.

There are some tools out there which make testing docker images easy:
https://medium.com/@aelsabbahy/tutorial-how-to-test-your-docker-image-in-half-a-second-bbd13e06a4a9
https://medium.com/@rajatvig/test-driven-development-of-docker-images-with-goss-3b6d23a16eaf
https://github.com/aelsabbahy/goss#manual-installation