Use export DISABLE_FIREWALL=1 before running install (I think the firewall can be made to work, but I didn’t attempt it).
With ubuntu-14.04-x86_64-minimal, I get this message on container start: resolvconf: Error: /run/resolvconf/interface either does not exist or is not a directory. Subsequently, DNS in the machine doe s not work because /etc/resolv.conf is empty, and any OpenVZ –nameserver options are not applied. This is because mail-in-a-box installs the resolv.conf package, and per /etc/vz/dists/scripts/set_dns.sh, if that package is install, it will be used to set the nameservers. Going through dpkg-reconfigure resolvconf there seem to be a bunch of approaches, though the interworkings of the whole system don’t really understand, but what worked for me was making sure that /etc/resolv.conf is a symlink to /run/resolvconf/resolv.conf, and that /etc/resolvconf/resolv.conf.d/tail contained my nameserver x.x.x.x instruction.
Update November 2016: During an upgrade, I ran into the DNS problem again. This time, the fix was to add nameserver instruction to /etc/resolvconf/resolv.conf.d/base, which ultimately survived the upgrade process. It seems that this error occuring during upgrade is somewhat distinct from the previous DNS Error, which I see *after* the install or upgrade.