I remember running VMs in VMs when I was younger and wondered how far I could go before my computer would implode. Infinite recursion is amazing. Let me rephrase that; infinity is amazing.
This is one of the <i>many</i> potential applications of the new "-privileged" flag of Docker. This is very exciting, because it means that we will soon run Docker CI within Docker itself (instead of ad-hoc VMs), among other things!
Mostly-unrelated question: could one build a desktop Linux distribution around Docker? Or rather, how difficult would it be? I assume OS X's application sandboxing uses a technology similar to LXC.
It is nice to see, especially because fast moving project is always an enjoyable thing to observer. As a sidenote, to increase overall awareness, FreeBSD has capability for hierarchical Jails (Jails inside of Jails) for a while.