The agent version was always showing 0.1.0 because build.rs tried to
run 'git describe' inside the Docker build, but .git directory wasn't
copied into the image.
Changes:
- Add VERSION build arg to Dockerfile
- Pass VERSION as env var to cargo build commands
- Update GitLab CI to pass --build-arg VERSION
- Modify build.rs to prefer BUILD_VERSION env var over git commands
This ensures the version displayed by the agent matches the Docker
image tag it was built with.
