I was excited to learn that Docker now officially supports rasbian on the Raspberry Pi 3.

Keen to try it out, I found Docker’s handy one-liner to install docker-engine on my RPi.
Warning: There’s an inherent risk whenever you pipe an arbitrary shell script into your bash interpreter.
I recommend curling the script into a file, first, so you can sanity check the commands before they’re executed on your box.

curl -sSL https://get.docker.com/ > script.sh
cat script.sh

TLDR: The shell script adds the docker repository to your list of aptitude sources, runs apt-get update, followed by the install command, apt-get install docker-engine.
If you’re happy that the script looks reasonable, make it executable and kick it off:

chmod +x script.sh
./script.sh

For me, the installation appeared to complete successfully, until it processed the triggers for systemd, the raspbian manager responsible for actually starting the docker process. Inspection of the logs showed that docker was failing to start, complaining about no unavailable network.

Jan 24 23:47:13 cavepi dockerd[9310]: Error starting daemon: Error initializing network controller: list bridge addresses failed: no available network
Jan 24 23:47:13 cavepi systemd[1]: docker.service: main process exited, code=exited, status=1/FAILURE
Jan 24 23:47:13 cavepi systemd[1]: Failed to start Docker Application Container Engine.
Jan 24 23:47:13 cavepi systemd[1]: Unit docker.service entered failed state.

After Googling around, I found several reports of similar (though not identical) errors, that had been traced back to openvpn having been running, at the time of install.
Indeed, I did have an openvpn server running at the time of install, so I tried halting the openvpn server, uninstalling docker-engine (plus its dependencies) from my RPi, and kicking off the installer again:

sudo systemctl stop openvpn
sudo apt-get remove --purge docker-engine
sudo apt-get autoremove
./script.sh

Unfortunately, even with the openvpn server disabled, I still faced the same error message:

Error starting daemon: Error initializing network controller: list bridge addresses failed: no available network

Eventually, I discovered that, despite passing the --purge flag to apt-get remove, some files had stuck around in the /var/lib/docker directory.
I believe these files still had a reference to my network from when the openvpn (tun0) interface was present, which meant openvpn was still biting me.
Alas, removing the rogue directory, and then repeating the steps to purge and clean install docker, resulted in a successful install 🎉.

rm -fr /var/lib/docker
sudo apt-get remove --purge docker-engine
sudo apt-get autoremove
./script.sh

I hope this helps someone else, out there.