Add Dockerfile instruction for private UIDs

This adds a Dockerfile instruction called PRIVATEUIDS which indicates
the range of host UIDs to use rather than just one default range of UIDs
(100k-110k) for all containers. It allows the user to specify which
contiguous set of 10k UIDs (referred to as a "bank" of UIDs) are mapped
into the container.  A cohesive group of containers that need to share
data through volumes must use the same bank.

--private-uids option is modified to take a parameter to specify the bank
as well.  The behavior of --uidmap and -x options remains the same.