In the common case where the user is using /var/lib/docker and an image with less than 60 layers, forking is not needed. Calculate whether absolute paths can be used and avoid forking to mount in those cases. Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan) Upstream-commit: c13a985fa1196a5ed782d5ac68a4bbb68dd529ca Component: engine