If you’re excited about playing with the swarm mode in the new 1.12 version of Docker, you can download the release candidates for Linux, Windows, Mac, but it’s not obvious where you can get an ARM compatible version to run on the Raspberry Pi.
The awesome guys over at Hypriot talk about running the 1.12 RC on the Pi, but their prebuilt OS images for the Pi don’t yet include the 1.12 RC version. I found this blog post however, which mentions a download location where you can pick up a .deb file and install it on the Pi with dpkg.
Downloaded, installed on my “to be” Pi cluster of 2 Pis, and up and running! Now to deploy something with swarm.
Editor's Note: That link Kevin Hooke mentions is to a post titled "How I setup a Raspberry Pi 3 Cluster Using The New Docker Swarm Mode In 29 Minutes" by Malcolm Jones. It is a very detailed article with links, pictures, and code and worth your time.