docs: suggest copy linking for GitLab integration guide (#11067)

## Summary

Hardlinking does not work in that context and raises a warning.

Setting the link mode to copy makes the warning go away.

## Test Plan

Tested on gitlab.com and our self-hosted GitLab instance.

Before changing the link mode:

https://gitlab.com/fgreinacher/uv-test/-/jobs/8986967570

After changing the link mode:

https://gitlab.com/fgreinacher/uv-test/-/jobs/8987026307.

:hammer_and_pick: with :heart: by
[Siemens](https://opensource.siemens.com/)

---------

Co-authored-by: Zanie Blue <[email protected]>

Author: fgreinacher

docs/guides/integration/gitlab.md

@@ -16,12 +16,11 @@ variables:
   UV_VERSION: 0.5
   PYTHON_VERSION: 3.12
   BASE_LAYER: bookworm-slim
-
-stages:
-  - analysis
+  # GitLab CI creates a separate mountpoint for the build directory,
+  # so we need to copy instead of using hard links.
+  UV_LINK_MODE: copy
 
 uv:
-  stage: analysis
   image: ghcr.io/astral-sh/uv:$UV_VERSION-python$PYTHON_VERSION-$BASE_LAYER
   script:
     # your `uv` commands