entry.sh requires GNU readlink and fails on systems with BSD readlink #298

entry.sh:29 resolves its directory with readlink -f "$0". BSD readlink (including the one shipped with macOS) has no -f option, so invoking the documented entry point there fails before any judge runs.

The README only requires Ruby and Bundler and does not state that GNU coreutils are required. Use a portable path-resolution method or document/install the GNU dependency explicitly. Add a smoke test using the supported host environments.