Add a testing procedure

The built image won't be pushed on production registry to avoid pushing a non-running image, even if successfully built.

The push on the production registry has to be manually triggered.

Also, we added a docker_test.sh script which helps to test the image on the testing VM. Basically, it will replace all picasoft.net in test.picasoft.net (including URLs in configuration, registry, traefik labels...), start from a fresh configuration, and reset volume.

As a consequence, if the service works, it means that it does not depend on old artifacts and that it is well designed.