nixos/tests/docker-tools-overlay: fix test

- change `virtualisation.docker.storageDriver` from deprecated `overlay`
to `overlay2`. Using `overlay` dockerd fails to start with error:
`ERROR: the overlay storage-driver has been deprecated and removed`
https://docs.docker.com/engine/deprecated/#legacy-overlay-storage-driver

nixos/tests/docker-tools-overlay.nix

@@ -12,7 +12,7 @@ import ./make-test-python.nix ({ pkgs, ... }:
       { ... }:
       {
         virtualisation.docker.enable = true;
-        virtualisation.docker.storageDriver = "overlay";  # defaults to overlay2
+        virtualisation.docker.storageDriver = "overlay2";
       };
   };