nixosTests.docker-tools-nix-shell: Fix test

This test fails because the VM doesn't have enough disk space. Inceasing
it by almost one GB makes the test pass.

nixos/tests/docker-tools-nix-shell.nix

@@ -17,7 +17,7 @@ in
       { ... }:
       {
         virtualisation = {
-          diskSize = 3072;
+          diskSize = 4000;
           docker.enable = true;
         };
       };