chore: add test and docs for handling nix store paths in global variables

docs/index.md

@@ -9,3 +9,14 @@ This project provides a Nix flake module that allows you to generate your `.gitl
 - **Modularity:** Define and manage your CI configurations in a structured and modular way using Nix modules, making it easier to share and reuse CI logic across multiple projects.
 
 This documentation will guide you through setting up and using Nix GitLab CI for your projects.
+
+## Warnings
+
+To save you from frantically searching these docs if something doesn't work as expected, here are the most important warnings ;)
+
+!!! warning
+
+    Do not put Nix store paths into global/pipeline variables. They will simply be passed through,
+    resulting in bad portability (if two runners have different archs for example, one cannot find the path).
+    If you need any Nix store path in env variables, always do it on the job level, there
+    it will automatically be computed at runtime, thus will always work no matter which runner it runs on.