systemd's ebuild deploys systemd-nspawn, and this service's .service file, /usr/lib/systemd/system/systemd-nspawn\@.service, explicitly needs the directory /var/lib/container/ to exist to function. Reproducible: Always
systemd has deprecated /var/lib/containers/ in favor of /var/lib/machines/ http://lists.freedesktop.org/archives/systemd-devel/2015-February/028447.html
If I'm reading the mailing list post properly, the directory will now be created at boot by new enough versions of systemd. tmpfiles gained support for a new "v" line type for creating btrfs subvolumes. If the underlying file system is a legacy file system, this automatically degrades to creating a normal directory. Among others /var/lib/machines is now created like this at boot, should it be missing. It's possible this bug might not be needed anymore
this should be ok with 225