Earlier this week, I had breakfast with a colleague of mine from Rancher. Rancher is a great "orchestrator" for Docker containers. I have recommended and used them in production environments.
Containers - one of the hottest technologies in the last year - is a much more efficient form of virtualization than traditional "hardware" virtualization (think VMWare or Xen), while providing a superior application distribution model.
The challenge is that while the native Docker tools are pretty good for managing individual servers with containers, managing more than a few containers, let alone across more than a few servers, becomes impossibly complex.