Add Docker Compose development setup

[rdhyee]

Summary

• add a Dockerfile, Compose stack, and Docker-specific Django settings for local development
• document the Docker quick start in the README
• fix clean image builds by installing g++ for libsass, using /opt on PYTHONPATH, and trimming local build-context noise

Testing

• docker compose build web
• docker compose up -d web
• docker compose exec -T web python manage.py check --settings=regluit.settings.docker