Arvados

Before downloading & installing the image for the current container, crunch-run should:

• Check current usage (size of docker images already present on the worker)
• Check whether the image for the current container is already installed; if not, add the estimated image size to the current usage figure
• Check whether any other crunch-run processes are running (if so, skip cleanup to avoid races where an image is deleted between "download" and "start" -- or handle the race in some other way)
• Delete old images as needed to ensure current usage ≤ maximum
• Don't delete images needed for the current container

This feature eliminates the need for the docker-cleaner service, so we also need to

• Remove docker-cleaner from the install docs
• Add "uninstall docker-cleaner" to the upgrade notes
• Remove docker-cleaner parts from package build/test scripts
• Remove docker-cleaner from the source tree

Ops can temporarily disable cleanup by putting the worker in "hold" state: new containers won't be scheduled, so cleanup won't run.