containers: deny networkmanager from managing the ve-* and vb-* NICs

Without this, when you've enabled networkmanager and start a
nixos-container the container will briefly have its specified IP
address but then networkmanager starts managing it causing the IP
address to be dropped.

nixos/modules/virtualisation/containers.nix

@@ -726,6 +726,11 @@ in
 
     networking.dhcpcd.denyInterfaces = [ "ve-*" "vb-*" ];
 
+    services.udev.extraRules = optionalString config.networking.networkmanager.enable ''
+      # Don't manage interfaces created by nixos-container.
+      ENV{INTERFACE}=="v[e,b]-*", ENV{NM_UNMANAGED}="1"
+    '';
+
     environment.systemPackages = [ pkgs.nixos-container ];
   });
 }