boot2docker is a lightweight Linux distribution based on Tiny Core Linux made specifically to run Docker containers. It runs completely from RAM, weighs ~27MB and boots in ~5s (YMMV).
See Frequently asked questions for more details.
/var/lib/docker
Download the installers for Mac OS X and Windows.
Check out the latest use and install instructions for Mac OS X and Windows
To save and share container images, automate workflows, and more sign-up for a free Docker Hub account.
Go to How to build for Documentation on how to build your own boot2docker ISO.