I used the official manual as a base: docs.docker.com/engine/installation/linux/docker-ce/debian
Step 1
aptitude remove docker docker-engine docker.io
aptitude install apt-transport-https ca-certificates curl gnupg2
Step 2
I use apt-get instead of aptitude to overcome the «policykit-1 : Depends: libpam-systemd but it is not installable» issue.
apt-get install software-properties-common
Step 3
curl -fsSL https://download.docker.com/linux/$(. /etc/os-release; echo "$ID")/gpg | sudo apt-key add -
Step 4 (optional)
add-apt-repository \
"deb [arch=amd64] https://download.docker.com/linux/$(. /etc/os-release; echo "$ID") \
$(lsb_release -cs) \
stable"
This step adds the following lines to the /etc/apt/sources.list file:
deb [arch=amd64] https://download.docker.com/linux/debian stretch stable
deb-src [arch=amd64] https://download.docker.com/linux/debian stretch stable
So use it only if you do not have these lines yet.
See also: My /etc/apt/sources.list for Discourse in Debian 9 (Stretch).
Step 5
aptitude update && aptitude install docker-ce
Step 6 (verification)
docker run hello-world