1. Upgrade Docker Image, Ubuntu 20.04 & Docker-CE 19.03.12. Please note: the previous version doesn't support Ubuntu 20.04
2. Make A DOCKER-USER -j ufw-user-forward as the first rule of DOCKER-USER, then we can use ufw to manage connections from containers.
3. Add unit test cases to this project, using Bach Testing Framework, check the ./tests folder.
4. Set up a basic testing environment by Vagrant, just run the command vagrant up in the project folder.

Test passed with Docker Swarm:

• Ubuntu 16.04 && Docker-CE 18.06
• Ubuntu 20.04 && Docker-CE 19.03.12