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

[Copilot]

Pull request overview

This PR suppresses uv’s recurring hardlink fallback warning in the GitHub Action container by configuring uv to use copy mode.

Changes:

• Set UV_LINK_MODE=copy in the Dockerfile to avoid hardlink attempts (and the associated warning) when cache and target directories are on different filesystems.

Show a summary per file

File	Description
Dockerfile	Adds ENV UV_LINK_MODE=copy so uv run uses copy mode and stops emitting the hardlink fallback warning.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 1

[Copilot]

This introduces a new environment variable (UV_LINK_MODE) in the action image. Per the repo guideline to document environment variable changes in README.md, please add a short note explaining why this is set (suppress hardlink warning when cache/venv are on different filesystems) and its impact (forces copy mode).