That new option allows us to remove the crutch of pretending to be a
container.
Pretending to be a container had a couple drawbacks, they are now
irrelevant.
The dummy outputs could be re-worked to instead use out own stage-1
infra, but I prefer using dummies for now, for better insulation.
Otherwise, passing configuration to the NixOS modules system would
become harder, for "outside-in" builds, and would make it harder to
integrate into a nixos-rebuild type build.
