It's a dockerized polyfill-service, which could help you run a self-hosted https://polyfill.io.
It's built on GitHub Actions automatically everyday when there's a new release of polyfill-service.
By default it listens port 80 in container. You could start it with:
docker run -d --name polyfill-service --restart unless-stopped -p 8080:80 menci/polyfill-service