nixos-containers: Add support for custom nixpkgs argument

nixos/modules/virtualisation/containers.nix

@@ -463,10 +463,15 @@ in
                 A specification of the desired configuration of this
                 container, as a NixOS module.
               '';
-              type = lib.mkOptionType {
+              type = let
+                confPkgs = if config.pkgs == null then pkgs else config.pkgs;
+              in lib.mkOptionType {
                 name = "Toplevel NixOS config";
-                merge = loc: defs: (import ../../lib/eval-config.nix {
+                merge = loc: defs: (import (confPkgs.path + "/nixos/lib/eval-config.nix") {
                   inherit system;
+                  pkgs = confPkgs;
+                  baseModules = import (confPkgs.path + "/nixos/modules/module-list.nix");
+                  inherit (confPkgs) lib;
                   modules =
                     let
                       extraConfig = {
@@ -515,6 +520,15 @@ in
               '';
             };
 
+            pkgs = mkOption {
+              type = types.nullOr types.attrs;
+              default = null;
+              example = literalExample "pkgs";
+              description = ''
+                Customise which nixpkgs to use for this container.
+              '';
+            };
+
             ephemeral = mkOption {
               type = types.bool;
               default = false;

nixos/tests/all-tests.nix

@@ -53,6 +53,7 @@ in
   consul = handleTest ./consul.nix {};
   cockroachdb = handleTestOn ["x86_64-linux"] ./cockroachdb.nix {};
   containers-bridge = handleTest ./containers-bridge.nix {};
+  containers-custom-pkgs.nix = handleTest ./containers-custom-pkgs.nix {};
   containers-ephemeral = handleTest ./containers-ephemeral.nix {};
   containers-extra_veth = handleTest ./containers-extra_veth.nix {};
   containers-hosts = handleTest ./containers-hosts.nix {};

nixos/tests/containers-custom-pkgs.nix

@@ -0,0 +1,42 @@
+# Test for NixOS' container support.
+
+import ./make-test-python.nix ({ pkgs, lib, ...} : let
+
+  customPkgs = pkgs // {
+    hello = pkgs.hello.overrideAttrs(old: {
+      name = "custom-hello";
+    });
+  };
+
+in {
+  name = "containers-hosts";
+  meta = with lib.maintainers; {
+    maintainers = [ adisbladis ];
+  };
+
+  machine =
+    { ... }:
+    {
+      virtualisation.memorySize = 256;
+      virtualisation.vlans = [];
+
+      containers.simple = {
+        autoStart = true;
+        pkgs = customPkgs;
+        config = {pkgs, config, ... }: {
+          environment.systemPackages = [
+            pkgs.hello
+          ];
+        };
+      };
+
+    };
+
+  testScript = ''
+    start_all()
+    machine.wait_for_unit("default.target")
+    machine.succeed(
+        "test $(nixos-container run simple -- readlink -f /run/current-system/sw/bin/hello) = ${customPkgs.hello}/bin/hello"
+    )
+  '';
+})