Oust WorkerVersion Docker image artifacts

https://redmine.teklia.com/issues/5720

Please remove the WorkerVersion.docker_image foreign key, which points to an artifact, and the docker_image_name property. These were used by Git imports to provide a Docker image that was built within an Arkindex process, but we now only rely on the docker_image_iid string, which points to the tag of an image hosted somewhere else.

The check constraint for a Docker image now requires that docker_image_iid is set at all times. This may require a data migration that updates the versions that do not fill this constraint to an Error state.

The arkindex cleanup command can now clean up all processes that had artifacts assigned as the Docker image of a WorkerVersion.