environment not in the chroot from untrusted files.
See also OpenVZ a3f732ef75/src/enter.c (L227-L234)
Signed-off-by: Justin Cormack <justin.cormack@docker.com>
(cherry picked from commit cea6dca993c2b4cfa99b1e7a19ca134c8ebc236b)
Signed-off-by: Tibor Vass <tibor@docker.com>