Fork the postfix cookbook to work around a bug

I ran into the issue described in
https://github.com/sous-chefs/postfix/issues/148
and couldn't figure out a way to work around it without forking it.

cookbooks/postfix/.markdownlint-cli2.yaml

@@ -3,3 +3,5 @@ config:
   line-length: false # MD013
   no-duplicate-heading: false # MD024
   reference-links-images: false # MD052
+ignores:
+  - .github/copilot-instructions.md