It occurred to me that NixOS might have a hard time doing something like this:

• create a 6 node Docker Swarm cluster, where 3 of the nodes are managers

You must’ve misunderstood. This sort of thing is exactly what NixOS excels at!

In the Nixpkgs CI, we often spin up multiple VMs in a single test to ensure things like networking between machines work as expected, all using relatively straight forward NixOS primitives.

I do not believe Docker Swarm specifically is supported (it’s a proprietary platform, so community interest understandably isn’t very high) but near same configurations with small differences are trivial to do with NixOS modules and they are a generic mechanism (can even be used to i.e. configure Android images and many other things).

I can see NixOS doing a fine job of templating a standard build that includes the Docker engine.

Note that NixOS’ module system is far far more powerful than simple templating.