I've been looking at creating a CentOS-5 docker image, and it's taking shape.
Currently my largest concern with the image is that it requires selinux to be disabled on the host, as the image currently lacks a patch similar to what was required for CentOS-6 [1].
If you would like to test this current iteration of this image, you can get it from http://people.centos.org/jperrin/docker/
The kickstart used to create the c5 docker image can be found in the https://github.com/CentOS/sig-cloud-instance-build/ repository.
Feedback is welcome.
[1] http://lists.centos.org/pipermail/centos-devel/2014-May/010345.html