dockerinit: propagate "container" env variable from lxc

Lxc (and libvirt) already set the "container" env variable
appropriately[1], so just use that.

[1] http://www.freedesktop.org/wiki/Software/systemd/ContainerInterface/
Merge pull request #3202 from jpoimboe/libvirt-prereq-env

dockerinit: propagate "container" env variable from lxc