Remove obsolete fields from worker configuration parser

Remove

• docker.build
• docker.context
• docker.image
• docker.environment

from the worker configuration parser.

Refs https://redmine.teklia.com/issues/6362

changed milestone to %Arkindex 1.6.1

added P3 Quick Win labels

changed the description

assigned to @mblanco

mentioned in merge request !277 (merged)

closed with merge request !277 (merged)