The runner runs jobs via act + DinD; `docker run -v "$PWD:/src"` from
inside the job container mounts the runner-job filesystem, not the
docker daemon's host fs, so the mount appears empty and `go test ./...`
fails with "directory prefix . does not contain main module".
Run tests in the same container that builds — same workspace, no mount.
Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>