Add integration test to build a docker file

We need an integration test (not triggered through tox by default), that builds a docker image through the WorkerVersion code. Api calls to Arkindex are mocked, but docker calls are real.

It should run in a dedicated task on CI, using a docker service (like our CI docker builds).

1. add a file in integration/test_worker_version.py
2. add a method test_docker_build() that runs when the module is executde with Python
3. in that method, configure a BuildWorkerVersion instance to be able to execute the build_image and save_image on a dummy Dockerfile
4. once it works locally, add a new task named integration-tests (test stage) in the Gitlab CI, with a docker service dependency (look at .build-job...) that simply calls the script.