Rely on docker.command to know the worker's command

• docker.command becomes mandatory (update documentation)
• update Dockerfile cookiecutter template to remove the CMD statement

Refs https://redmine.teklia.com/issues/7727

changed milestone to %0.4.0

added P3 Quick Win labels

assigned to @ebardou

mentioned in merge request !566 (merged)

closed with merge request !566 (merged)