enable cross compilation

nix-builder/Dockerfile

@@ -3,7 +3,7 @@ FROM nixos/nix
 RUN nix-channel --add https://nixos.org/channels/nixos-unstable nixpkgs \
     && nix-channel --update
 
-RUN nix-env -iA nixpkgs.qemu nixpkgs.nixos-generators
+RUN nix-env -iA nixpkgs.pkgsCross nixpkgs.nixos-generators
 
 RUN mkdir /workspace
 

workspace/build.sh

@@ -1,6 +1,5 @@
 
 nix-build '<nixpkgs/nixos>' \
   -A config.system.build.sdImage \
-  --argstr system armv7l-linux \
   --arg pkgs 'import <nixpkgs> { crossSystem = { system = "armv7l-linux"; }; }' \
-  -I nixos-config=./rpi2.nix
+  -I nixos-config=./rpi2.nix

workspace/rpi2.nix

@@ -40,4 +40,9 @@
   
   # Set system version (avoid warnings)
   system.stateVersion = "25.05";
+
+  # Enable cross-compilation
+  nixpkgs.crossSystem = {
+    system = "armv7l-linux";
+  };
 }