I'm sorry, I do not have any docker experience
I suppose that this procedure installs systemd skripts only, but XBian is using upstart
 
I downloaded the package from here:
https://download.docker.com/linux/debian/dists/stretch/pool/stable/armhf/
And it contains an 
/etc/init.d/docker start/stop script:
Terminal
xbian@avr ~ $ dpkg-deb -c docker-ce_18.09.4~3-0~debian-stretch_armhf.deb
drwxr-xr-x root/root         0 2019-03-27 19:45 ./
drwxr-xr-x root/root         0 2019-03-27 19:45 ./etc/
drwxr-xr-x root/root         0 2019-03-27 19:45 ./etc/default/
-rw-r--r-- root/root       654 2019-03-27 19:01 ./etc/default/docker
drwxr-xr-x root/root         0 2019-03-27 19:45 ./etc/init/
-rw-r--r-- root/root      1875 2019-03-27 19:01 ./etc/init/docker.conf
drwxr-xr-x root/root         0 2019-03-27 19:45 ./etc/init.d/
-rwxr-xr-x root/root      3843 2019-03-27 19:01 ./etc/init.d/docker
drwxr-xr-x root/root         0 2019-03-27 19:45 ./lib/
drwxr-xr-x root/root         0 2019-03-27 19:45 ./lib/systemd/
drwxr-xr-x root/root         0 2019-03-27 19:45 ./lib/systemd/system/
-rw-r--r-- root/root      1684 2019-03-27 19:45 ./lib/systemd/system/docker.service
-rw-r--r-- root/root       197 2019-03-27 19:45 ./lib/systemd/system/docker.socket
drwxr-xr-x root/root         0 2019-03-27 19:45 ./usr/
drwxr-xr-x root/root         0 2019-03-27 19:45 ./usr/bin/
-rwxr-xr-x root/root    387564 2019-03-27 19:45 ./usr/bin/docker-init
-rwxr-xr-x root/root   2798536 2019-03-27 19:45 ./usr/bin/docker-proxy
-rwxr-xr-x root/root  50184184 2019-03-27 19:45 ./usr/bin/dockerd-ce
drwxr-xr-x root/root         0 2019-03-27 19:45 ./usr/share/
drwxr-xr-x root/root         0 2019-03-27 19:45 ./usr/share/doc/
drwxr-xr-x root/root         0 2019-03-27 19:45 ./usr/share/doc/docker-ce/
-rw-r--r-- root/root      1576 2019-03-27 19:01 ./usr/share/doc/docker-ce/README.md
-rw-r--r-- root/root       146 2019-03-27 19:45 ./usr/share/doc/docker-ce/changelog.Debian.gz
drwxr-xr-x root/root         0 2019-03-27 19:45 ./var/
drwxr-xr-x root/root         0 2019-03-27 19:45 ./var/lib/
drwxr-xr-x root/root         0 2019-03-27 19:45 ./var/lib/docker-engine/
-rw-r--r-- root/root       140 2019-03-27 19:45 ./var/lib/docker-engine/distribution_based_engine-ce.json
So below one should trigger it to start at boot:
Code:
sudo update-rc.d docker defaults
 
 
I've installed and run Docker successfully on 2 different XBian systems, so I made a script to install it:
Code:
#!/bin/sh
#
# Script to install and check docker on XBian system
# Commands token from here: 
#     https://docs.docker.com/install/linux/docker-ce/debian/#install-using-the-repository
#  
   
# Update the apt package index:
sudo apt-get update
   
# Install packages to allow apt to use a repository over HTTPS:
sudo apt-get install \
    apt-transport-https \
    ca-certificates \
    curl \
    gnupg2 \
    software-properties-common
   
# Add Docker’s official GPG key:
curl -fsSL https://download.docker.com/linux/debian/gpg | sudo apt-key add -
   
# Set up the stable repository:
sudo add-apt-repository \
   "deb [arch=armhf] https://download.docker.com/linux/debian \
   $(lsb_release -cs) \
   stable"
   
# Update the apt package index:
sudo apt-get update
   
# Install the latest version of Docker CE and containerd, or go to the next step to install a specific version:
sudo apt-get install docker-ce docker-ce-cli containerd.io
   
# Check, of docker is already running, it should be
   
status docker
   
# Verify that Docker CE is installed correctly by running the hello-world image.
sudo docker run hello-world
 
On one machine, I'm running PiHole now in a Docker container 
 
You can cut and paste the script, store it make it executable and run it with sudo. Btw, Docker daemon is started automatically on reboot