Revert "nixos/tests/docker-tools*: remove useless formatter"

Annoyed with the interference of the python formatting of
generated code (see #72964), I took matters into my own hands
as maintainer of dockerTools.

Afterwards, I've created a PR, hoping to unstuck the discussion.

@aszlig took notice and thanks to his python ecosystem knowledge,
the testing efforts of @blaggacao and @Ma27, and a sense of
shared suffering and comraderie we were able to change the
situation for the better in #122201.

Now, we have a proper linter that actually helps contributors,
so it's time to turn it back on again.

I'm glad we could make it happen this quickly!

Thanks!

This reverts commit 4035049af3e45554ffc4d8b4c30fd43ae9cd328a.

nixos/tests/docker-tools-cross.nix

···
             "docker rmi ${hello2.imageName}",
         )
   '';
-
-  # Remove when the formatter has been removed and a linter has been added
-  skipLint = true;
 })

nixos/tests/docker-tools-overlay.nix

···
       # drw------- 99 0 0 100 Apr 14 11:36 /nix/store
       docker.succeed("docker run --rm -u 1000:1000 ${pkgs.dockerTools.examples.bash.imageName} bash --version")
     '';
-
-  # Remove when the formatter has been removed and a linter has been added
-  skipLint = true;
 })

nixos/tests/docker-tools.nix

···
             "docker run --rm ${examples.layeredImageWithFakeRootCommands.imageName} sh -c 'stat -c '%u' /home/jane | grep -E ^1000$'"
         )
   '';
-
-  # Remove when the formatter has been removed and a linter has been added
-  skipLint = true;
 })