nspawn + btrfs is my preferred solution to the "50 containers" problem. The incantation you want is:

    systemd-nspawn --template="/path/to/subvolume" <other args>

This creates a copy-on-write snapshot of the subvolume you supply, which is instantaneous. The --ephemeral flag can be used instead if you want the guest to be able to modify the base filesystem but you do not want those changes to persist across container boots.

Can someone describe what advantages rkt gives you over plain nspawn containers?