feat(nix): add docker

Signed-off-by: Seongmin Lee <[e-mail]>

nix/darwin.nix

···
     enable = true;
     casks = [
       "discord"
+      "docker"
       "ghostty"
       "raycast"
       # there are also pkgs.sensible-side-buttons (original version) which is unfree license

nix/flake.nix

···
         pkgs.brave
         pkgs.iina
         pkgs.macmon
+        pkgs.docker
       ] else []);
     };
   };