Use image to choose runner

Images should be used in the same way as tags when selecting a runner. If a runner does not allow the image for a job then do not send the task to it. This would require runners publishing to the coordinator their list of allowed_images and the coordinator checking these before assigning a task to a runner. If no runners are available with the required image in their allowed list, then the task should be set to pending (as it would with no runners with matching tags being available).

It seems silly/redundant that for every type of image I need to specify a tag to be able to determine which runners the job can run on, I would rather use tags for specific features of the image (volumes, devices, configurations, etc. available to the image).