Debian ships with an old version of Docker, and the official installation instructions for Docker on Debian are a bit dubious (run an entirely untrusted shell script as root! yay!), not to mention error-prone, and result in a completely non-functional Docker installation on Debian, thanks to aufs being deprecated.

So these instructions should make it possible to install Docker 1.12 on Debian Stretch. At least for now. Stretch is a moving target, so these instructions may become quickly outdated.

1. Install the Docker archive keyring:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 58118E89F3A912897C070ADBF76221572C52609D

2. Configure apt:

sudo echo "deb https://apt.dockerproject.org/repo debian-stretch main" > /etc/apt/sources.list.d/docker.list

3. Install the package:

   If you had an old, Debian-official version of docker installed, you should remove it first:

   sudo aptitude purge docker
   

   Then install the new one:

   sudo aptitude install docker-engine
   

4. Configure the service:

   This is the most complicated part, which required some investigation. You’ll need to edit the systemd service file, to tell it to use an environment file. So open /lib/systemd/system/docker.service. You will then make two changes, both in the [Service] section.

    [Service]
   +EnvironmentFile=/etc/default/docker
    Type=notify
    # the default is not to use systemd for cgroups because the delegate issues still
    # exists and systemd currently does not support the cgroup feature set required
    # for containers run by docker
   -ExecStart=/usr/bin/dockerd -H fd://
   +ExecStart=/usr/bin/dockerd $DOCKER_OPTS
    ExecReload=/bin/kill -s HUP $MAINPID
   

   First, add the line EnvironmentFile=/etc/default/docker at the top of the [Service] section.

   Second, on the ExecStart= line, replace -H fd:// with $DOCKER_OPTS.

   Together, these changes tell the docker service to read the /etc/default/docker file for environment variables.

   Then, edit (or create) the file /etc/default/docker, and make sure it contains the following, uncommented line:

   DOCKER_OPTS="--storage-driver=overlay -H fd://"
   

   This tells dockerd to use the overlay filesystem, instead of the obsolete default of aufs.

5. Reload the systemd configuration:

   sudo systemctl daemon-reload
   

6. Start docker:

   sudo /etc/init.d/docker start
   

That should do it! Please let me know if you experience any problems, or I need to update anything here.