Arvados

Currently arv-keepdocker works by subprocessing the docker CLI tool and parsing the output. Port it to stop doing that, and instead use the docker-py module to talk to the Docker daemon directly over the REST API and get objects back.

Reasons to do this:

• The CLI has changed interfaces before in ways that break arv-keepdocker. (e.g., #4196) This would be more resilient.
• The client can specify which version of the API it wants to talk, making it even more resilient as Docker itself gets upgraded (which is often).
• IPC is always rickety. This would be more resilient to different user environments and enable better error reporting.
• docker-py is maintained by the Docker team, and we already use it with no major isseus in dockercleaner, so it's a pretty safe dependency.

The branch should only rewire arv-keepdocker internally. You'll also need to declare the dependency in setup.py and run-build-packages.sh. No UI changes should be needed.