The LISTEN_FDS environment variable defines the number of
file descriptors that should be inherited by the container,
in addition to stdio.
The LISTEN_FDS environment variable is defined in the OCI spec
and used to support socket activation.

Refs #3845
Signed-off-by: Ruben Jenster <r.jenster@drachenfels.de>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>