fix: suppress uv hardlink warning with UV_LINK_MODE=copy

[zkoppert]

Proposed Changes

When running as a GitHub Action, the uv cache and target .venv directory end up on different filesystems (container overlay vs host mount), causing uv to emit a noisy warning on every run:

warning: Failed to hardlink files; falling back to full copy. This may lead to degraded performance.
         If the cache and target directories are on different filesystems, hardlinking may not be supported.
         If this is intentional, set `export UV_LINK_MODE=copy` or use `--link-mode=copy` to suppress this warning.

Adding ENV UV_LINK_MODE=copy to the Dockerfile tells uv to use copy mode directly, suppressing the warning.

This is the same fix applied in github-community-projects/pr-conflict-detector#65.

	Before	After
Hardlink warning	Printed on every run	Suppressed

Testing

• docker build - image builds successfully with the new env var

Readiness Checklist

Author/Contributor

• If documentation is needed for this change, has that been included in this pull request
• run make lint and fix any issues that you have introduced
• run make test and ensure you have test coverage for the lines you are introducing