this started with emily pointing out to me that it's possible to
generate IP addresses for containers in Nix (hence no need to worry
about ever having collisions, as we had before), but then I thought,
hey, while I'm at it, I can also write a little container module so we
have a little less repetition in our configs in general (and a more
reasonable place for our custom evalConfig than just keeping it around
in flake.nix).
See the option descriptions in modules/containers.nix for further
details.
Apart from giving all containers a new IP address (and also shiny new
IPv6 addresses), this should be a no-op for the actual built system.
