Set Docker image artifact for a worker version

A WorkerVersion needs a new nullable Foreign Key towards ponos.Artifact, named docker_image in order to be used later on in workflows.

That field is null at first, but must be updated by the docker build task, on a successful build & upload.

This issue should have 2 MRs:

1. on the backend to add the field and support setting the field through the worker version update api
2. on the tasks to set that value after the artifact has been uploaded