To install docker on Debian 9, run following commands as user root.

apt-get -y install apt-transport-https ca-certificates curl software-properties-common
curl -fsSL https://download.docker.com/linux/debian/gpg | apt-key add -
apt-key fingerprint 0EBFCD88
add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/debian $(lsb_release -cs) stable"
apt-get update
apt-get install docker-ce

Start docker

service docker start

Test docker is working by running a container, we run alphine linux container, that is very small linux distribution.

docker run -it alpine /bin/sh

Example

root@boby:~# docker run -it alpine /bin/sh
Unable to find image 'alpine:latest' locally
latest: Pulling from library/alpine
627beaf3eaaf: Pull complete 
Digest: sha256:58e1a1bb75db1b5a24a462dd5e2915277ea06438c3f105138f97eb53149673c4
Status: Downloaded newer image for alpine:latest
/ # ls
bin    dev    etc    home   lib    media  mnt    proc   root   run    sbin   srv    sys    tmp    usr    var
/ #