They have official images for Ubuntu, Centos and Busybox, and unofficial ones for many more (Gentoo, Arch, Debian, OpenSUSE, and others).
See https://github.com/dotcloud/docker/wiki/Public-docker-images and https://index.docker.io/
See https://github.com/dotcloud/docker/wiki/Public-docker-images and https://index.docker.io/