docker: Fix docker image gc tracking
docker: Fix docker image gc tracking

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).

dvusboy/nomad,hashicorp/nomad,burdandrei/nomad,hashicorp/nomad,dvusboy/nomad,burdandrei/nomad,hashicorp/nomad,burdandrei/nomad,burdandrei/nomad,burdandrei/nomad,hashicorp/nomad,dvusboy/nomad,dvusboy/nomad,dvusboy/nomad,dvusboy/nomad,burdandrei/nomad,burdandrei/nomad,dvusboy/nomad,hashicorp/nomad,hashicorp/nomad