Docker doesn't yet have a release file for Ubuntu 17.10 (it should be coming soon though). As such Docker CE can be installed from the Zesty package as follows:
sudo apt-get install apt-transport-https ca-certificates curl software-properties-common
curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
sudo apt-key fingerprint 0EBFCD88
sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu zesty stable"
sudo apt-get update
sudo apt-get install docker-ce
Many thanks to levsthings