Use pre-built golangci/golangci-lint:v2.1.6 image for fast lint feedback
instead of installing golangci-lint from source on every build.
- Lint stage: runs fmt-check and lint using pre-built image
- Build stage: runs tests and compiles binaries
- COPY --from=lint forces BuildKit to execute the lint stage
- All images pinned by sha256 digest
- Runtime stage unchanged