Project: mesos
Issue MESOS-4757: Mesos containerizer should get uid/gids before pivot_root.
Currently, we call os::su(user) after pivot_root. This is problematic because /etc/passwd and /etc/group might be missing in container's root filesystem. We should instead, get the uid/gids before pivot_root, and call setuid/setgroups after pivot_root.
Story points: 3