nixos/tests: fix docker test

The docker service is socket activated by default; thus,
`waitForUnit("docker.service")` before any docker command causes the
unit test to time out.

Instead, do `waitForUnit("sockets.target")` to ensure that sockets are
setup before running docker commands.

nixos/tests/docker.nix

···
   testScript = ''
     startAll;
 
-    $docker->waitForUnit("docker.service");
+    $docker->waitForUnit("sockets.target");
     $docker->succeed("tar cv --files-from /dev/null | docker import - scratchimg");
     $docker->succeed("docker run -d --name=sleeping -v /nix/store:/nix/store -v /run/current-system/sw/bin:/bin scratchimg /bin/sleep 10");
     $docker->succeed("docker ps | grep sleeping");