* docker: set force=true on remove image to handle images referenced by multiple tags
This PR changes our call of docker client RemoveImage() to RemoveImageExtended with
the Force=true option set. This fixes a bug where an image referenced by more than
one tag could never be garbage collected by Nomad. The Force option only applies to
stopped containers; it does not affect running workloads.
* docker: add note about image_delay and multiple tags
