Dynamic image loading to docker hosts

DockerDriver should load images dynamically to notebook hosts. Current behaviour is so that only when a new host is spawned, all images are uploaded to that and the scheduling/host selection assumes all images are present on all hosts. This causes operational overhead and delay when adding images as well scalability problems wrt. number of images

Start by locating DockerDriver and the scheduling or host-selection entry points, then trace where images are uploaded when a notebook host is created. Determine how the current assumption that every host has every image is enforced. Done means images can be loaded dynamically on demand without requiring all images on every host, with the relevant behavior covered by existing tests if available.

Written by the indexing model from the issue text.