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 UIDs when pulling the image and normalize when pushing

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

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