Fixup permissions of cache directories for user namespaces

Docker-DCO-1.1-Signed-off-by: Dinesh Subhraveti <dineshs@altiscale.com> (github: dineshs-altiscale)
Utilities required for user namespaces

Docker-DCO-1.1-Signed-off-by: Dinesh Subhraveti <dineshs@altiscale.com> (github: dineshs-altiscale)
Remap and normalize images for user namespaces

Images can be remapped and normalized with -x and -y options as follows:

docker commit $(docker run -dx ubuntu true) ubuntu-remapped
docker commit $(docker run -dy ubuntu-remapped true) ubuntu-normalized

Docker-DCO-1.1-Signed-off-by: Dinesh Subhraveti <dineshs@altiscale.com> (github: dineshs-altiscale)
Enable UID mapping by invoking the driver

Docker-DCO-1.1-Signed-off-by: Dinesh Subhraveti <dineshs@altiscale.com> (github: dineshs-altiscale)