Using flake output package in existing service

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  1. Name the argset that’s passed to the outputs functon of the consumer flake. Usually people call it inputs but the name doesn’t matter as long as you’re consistent.
  outputs =
    inputs @ {
  1. Pass the binding into your config via the specialArgs arg of lib.nixosSystem:
nixosConfiguratios.FOO = lib.nixosSystem {
  specialArgs = { inherit inputs; }
  # ...
};
  1. Use that module arg in your config
{ pkgs, inputs, ... }:
{
  services.caddy.package = inputs.caddy.packages.${pkgs.stdenv.hostPlatform.system}.default;
}

I’m not 100% familiar with caddy or its module, but that’s usually what I do for other modules.