nixos/mesos-slave: start after docker

If docker is enabled, start mesos-slave.service after docker.service
to avoid a race condition that could result in mesos-slave to fail
with "Failed to create docker: Timed out getting docker version"

nixos/modules/services/misc/mesos-slave.nix

···
     systemd.services.mesos-slave = {
       description = "Mesos Slave";
       wantedBy = [ "multi-user.target" ];
-      after = [ "network.target" ];
+      after = [ "network.target" ] ++ optionals cfg.withDocker [ "docker.service" ] ;
       path = [ pkgs.runtimeShellPackage ];
       serviceConfig = {
         ExecStart = ''