This fixes a bug where docker images may not be GCed. The cause of the
bug is that we track the task using `task.ID+task.Name` on task start
but remove on plain `task.ID`.
This haromize the two paths by using `task.ID`, as it's unique enough
and it's also used in the `loadImage` path (path when loading an image
from a local tarball instead of dockerhub).