virtualisation.docker: require docker.service for docker-prune.service

else docker-prune fails when docker is not launched. Adjusted the test as well

nixos/modules/virtualisation/docker.nix

···
         '';
 
         startAt = optional cfg.autoPrune.enable cfg.autoPrune.dates;
+        after = [ "docker.service" ];
+        requires = [ "docker.service" ];
       };
 
       assertions = [

nixos/tests/docker.nix

···
       { pkgs, ... }:
         {
           virtualisation.docker.enable = true;
+          virtualisation.docker.autoPrune.enable = true;
           virtualisation.docker.package = pkgs.docker;
 
           users.users = {