The uninstallation step mentions:
sudo apt-get purge -y docker-engine
sudo apt-get autoremove -y --purge docker-engine
sudo apt-get autoclean
sudo apt-get purge -y docker-engine docker docker.io docker-ce
sudo apt-get autoremove -y --purge docker-engine docker docker.io docker-ce
The above commands will not remove images, containers, volumes, or user created configuration files on your host. If you wish to delete all images, containers, and volumes run the following command
sudo rm -rf /var/lib/docker
Remove docker from apparmor.d:
sudo rm /etc/apparmor.d/docker
Remove docker group:
sudo groupdel docker