Build release binaries inside an Ubuntu 20.04 Docker container
to produce executables with lower GLIBC requirements, compatible
with older Linux distributions.
- Add Dockerfile.compat (Ubuntu 20.04 base, full depends rebuild)
- Add .dockerignore to exclude host build artifacts from context
- Add --linux-compat flag to build.sh with Docker build/extract/package
- Strip binaries inside container to avoid root ownership issues
