iso: copy input closourse into iso to avoide re-download

doc/start/iso.md

@@ -46,6 +46,18 @@ _Note: the [static link-local IPv6 address][staticLLA] and [MulticastDNS][mDNS]
 configured on the live installer. If you wish to enable [MulticastDNS][mDNS]
 for your environment, you ought to configure that in a regular [profile](../../profiles)._
 
+## ISO image nix store & cache
+
+The iso image holds the store to the live environment and _also_ acts as a binay cache
+to the installer. To considerably speed up things, the image already includes all flake
+`inputs` as well as the `devshell` closures.
+
+While you _could_ provision any machine with a single stick, a bespoke iso maximises
+those local cache hits.
+
+For hosts that don't differ too much, a common usb stick might be ok, whereas when
+there are bigger differences, a bespoke usb stick will be considerably faster.
+
 ## EUI-64 LLA & Host Identity
 
 The iso's IPv6 Link Local Address (LLA) is configured with a static 64-bit Extended
@@ -97,11 +109,6 @@ $ flk install NixOS --impure # use same host as above
 
 <!-- TODO: find out why --impure is necesary / PRs welcome! -->
 
-_Note: You _could_ install another machine than the one your iso was built for,
-but the iso doesn't carry all the necesary build artifacts so the target would
-start to build the missing parts on demand instead of substituting them from
-the iso itself._
-
 [manual]: https://nixos.org/manual/nixos/stable/index.html#sec-installation-partitioning
 [mDNS]: https://en.wikipedia.org/wiki/Multicast_DNS
 [NDP]: https://en.wikipedia.org/wiki/Neighbor_Discovery_Protocol

flake.nix

@@ -57,7 +57,7 @@
           overlay = import ./pkgs;
           overlays = lib.pathsToImportedAttrs (lib.pathsIn ./overlays);
 
-          lib = import ./lib { inherit nixos pkgs self; };
+          lib = import ./lib { inherit nixos pkgs self inputs; };
 
           templates.flk.path = ./.;
           templates.flk.description = "flk template";

lib/devos/devosSystem.nix

@@ -1,4 +1,4 @@
-{ lib, nixos, self, ... }:
+{ lib, nixos, self, inputs, ... }:
 
 { modules, ... } @ args:
 lib.nixosSystem (args // {
@@ -17,6 +17,10 @@ lib.nixosSystem (args // {
                 source = self;
                 target = "/devos/";
               }];
+              nix.registry = lib.mapAttrs (n: v: { flake = v; }) inputs;
+              isoImage.storeContents = [
+                self.devShell.${config.nixpkgs.system}
+              ];
               # confilcts with networking.wireless which might be slightly
               # more useful on a stick
               networking.networkmanager.enable = lib.mkForce false;