Fix for setuid race condition in LXC driver

This is a fix for a race condition in the LXC driver.  This is described
more in issue #6092.

Closes #6092

Docker-DCO-1.1-Signed-off-by: Shane Canon <scanon@lbl.gov> (github: scanon)
Merge pull request #6138 from NERSC/6092-setuidrace

Fix for setuid race condition in LXC driver