Containers are also part of Nix (which unlike Guix uses systemd). AFAIK, in Nix these are used for booting up an isolated environment (e.g. if you're running some binary which you perhaps would like to isolate).

But they are two different things: nix package manager -> managing dependencies, containers -> managing isolation.

By performing both at once, I feel systemd is not only breaking the Unix ethos (do one thing, and do it well); but it may eventually also creep into the package manager arena. Eventually, most userland will belong to systemd.