Add UV_LINK_MODE to Docker caching example (#7510)

As mentioned in #7509

docs/guides/integration/docker.md

@@ -301,10 +301,15 @@ A [cache mount](https://docs.docker.com/build/guide/mounts/#add-a-cache-mount) c
 improve performance across builds:
 
 ```dockerfile title="Dockerfile"
+ENV UV_LINK_MODE=copy
+
 RUN --mount=type=cache,target=/root/.cache/uv \
     uv sync
 ```
 
+Changing the default [`UV_LINK_MODE`](../../reference/settings.md#link-mode) silences warnings about
+not being able to use hard links since the cache and sync target are on separate file systems.
+
 If you're not mounting the cache, image size can be reduced by using the `--no-cache` flag or
 setting `UV_NO_CACHE`.