I like the instructions on the digitalocean.com but got tired of having to copy each command so put them all in a script that can be downloaded and run.

docker-install.sh

# Script for Unbuntu 18.04
# Once done, you should just be able to type docker... etc. 

sudo apt update

sudo apt install apt-transport-https ca-certificates

curl software-properties-commoncurl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -

sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu bionic stable"

sudo apt update

apt-cache policy docker-ce

sudo apt install docker-ce

sudo systemctl status docker

sudo usermod -aG docker ${USER}

su - ${USER}

id -nG