Allow specify the usage of a docker image on a specific agent node, in all the pipeline.

At the moment, agent only accepts one of label, node, docker at the time.

Could be something like:

agent { label 'my_node_label', docker 'my_docker_image'}

Use case would be that only the nodes with a specific label, have docker installed.

The only workaround I found was to assign a agent label at the top, and then inside of each stage, use agent docker.

This has the inconvinience that needs to be set on every stage.