Make VirtualBox a lighter build

Remove the VirtualBox GUI, see
https://search.nixos.org/options?channel=21.05&show=virtualisation.virtualbox.host.headless&from=0&size=50&sort=relevance&query=virtualbox

morph/grid/local/README.rst

@@ -5,10 +5,18 @@ Set up and use a network of local development VMs
 (The author of this documentation wasted a lot of time trying to get Vagrant to work with KVM/libvirt.
 Issues with networking that looked like guest misconfigurations vanished after changing to the better-tested combination of Vagrant and VirtualBox.)
 
+This requires `NixOS <https://nixos.org/>/`_.
+Nix without the OS will not work.
 
 Use the local development environment
 `````````````````````````````````````
 
+0. Add VirtualBox to your NixOs system configuration at ``/etc/nixos/configuration.nix``::
+
+    virtualisation.virtualbox.host.enable = true;
+    # Use VirtualBox installation without GUI and Qt dependency:
+    virtualisation.virtualbox.host.headless = true;
+
 1. Enter the morph local grid directory::
 
     cd morph/grid/local