Running LXC and Docker containers on a Jessie instance requires having memory cgroup support enabled.
Memory cgroups support requires editing /etc/default/grub and reboot:
If these changes were made to the default Jessie image offered in Labs it would make setting up MediaWiki-Vagrant and/or Docker containers easier for everyone.
Change 262838 had a related patch set uploaded (by Andrew Bogott):
Enable memory cgroups for labs debian instances
Change 262839 had a related patch set uploaded (by Andrew Bogott):
Labs jessie image: override the debian grub with a copy of our current puppetized grub defaults.
Change 262838 merged by Andrew Bogott:
Enable memory cgroups for labs debian instances
Change 262839 merged by Andrew Bogott:
Labs jessie image: override the debian grub with a copy of our current puppetized grub defaults.
Are you sure cgroup_enable=memory is needed nowadays? An lxc-checkconfig on an unmodified jessie system says "Cgroup memory controller: enabled".
swapaccount=1 is indeed needed (grep MEMCG_SWAP /boot/config-3.16.0-4-amd64 has "# CONFIG_MEMCG_SWAP_ENABLED is not set") as it generates some memory overheads and makes a difference only if you care about your cgroups using all of your swap.
It may indeed be cargo cult configuration. I think I came to this combined set of options based on Google searches for error messages I received when performing vagrant up operations on a jessie testing image. There is a note at https://wiki.debian.org/LXC#Prepare_the_host that seems to read as though memory cgroup support is still disabled by default in jessie but I may be reading this incorrectly and it may instead only apply to pre-jessie distributions. Some or all issues may be caused by the particular way that the Vagrant-LXC provider plugin manages an LXC container.