Per new policy: CI actions simply run 'docker build .'. The Dockerfile
now installs golangci-lint and runs 'make check' early in the build
process, so a successful docker build implies all checks pass.
- Dockerfile: add golangci-lint install and 'make check' before final build
- CI workflow: simplify to just 'docker build .' (no Go setup needed)
- Makefile targets unchanged
