This is useful for preventing CVE-2018-15664 where a malicious container
process can take advantage of a race on symlink resolution/sanitization.
Before this change chrootarchive would chroot to the destination
directory which is attacker controlled. With this patch we always chroot
to the container's root which is not attacker controlled.
Signed-off-by: Brian Goff <cpuguy83@gmail.com>
