Detect running in LXC container using new method

It seems LXC 2.0 switched to using the systemd container interface
specification and one can no longer detect LXC containers using
/proc/1/cgroup (but I am leaving the old method, so that people using
distros with older versions of LXC can still use Gitian).

https://www.freedesktop.org/wiki/Software/systemd/ContainerInterface/

target-bin/bootstrap-fixup.in

@@ -22,7 +22,7 @@ fi
 echo '127.0.1.1 gitian' >> /etc/hosts
 
 # If LXC
-if grep /lxc/gitian /proc/1/cgroup > /dev/null; then
+if grep /lxc/gitian /proc/1/cgroup > /dev/null || grep container=lxc /proc/1/environ > /dev/null; then
   adduser --disabled-password --gecos ${DISTRIB_NAME,,} --quiet ${DISTRIB_NAME,,} || true
   apt-get remove -y rsyslog || true
   dpkg-divert --local --rename --add /sbin/initctl