Containers: Use /etc/resolv.conf supplied by the host

This used to work with systemd-nspawn 203, because it bind-mounted
/etc/resolv.conf (so openresolv couldn't overwrite it). Now it's just
copied, so we need some special handling.

nixos/modules/virtualisation/container-config.nix

@@ -8,6 +8,8 @@ with lib;
 
     sound.enable = mkDefault false;
 
+    networking.useHostResolvConf = true;
+
     # Provide a login prompt on /var/lib/login.socket.  On the host,
     # you can connect to it by running ‘socat
     # unix:<path-to-container>/var/lib/login.socket -,echo=0,raw’.