https://docs.docker.com/engine/install/ubuntu/
20.x - focal
https://download.docker.com/linux/ubuntu/dists/focal/

sudo apt remove docker docker-engine docker.io containerd runc
sudo apt install apt-transport-https ca-certificates curl gnupg lsb-release
curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo gpg --dearmor -o /usr/share/keyrings/docker-archive-keyring.gpg
echo "deb [arch=amd64 signed-by=/usr/share/keyrings/docker-archive-keyring.gpg] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable" | sudo tee /etc/apt/sources.list.d/docker.list > /dev/null
sudo apt update
sudo apt install docker-ce docker-ce-cli containerd.io

...
https://docs.docker.com/engine/install/linux-postinstall
    sudo groupadd docker
...
sudo systemctl edit docker.service
    ! using nano editor, not a vim

storage
https://docs.docker.com/storage/storagedriver/overlayfs-driver/
sudo vim /etc/docker/daemon.json
{
  "storage-driver": "overlay2"
}

issues
https://www.digitalocean.com/community/questions/how-to-fix-docker-got-permission-denied-while-trying-to-connect-to-the-docker-daemon-socket
    ! just reboot