"Never upgrade a server again. Never update your code. Instead, create new servers, and throw away the old ones!"
That's the idea of immutable servers, or immutable infrastructure. This makes many things easier: rollbacks (you can always bring back the old servers), A/B testing (put old and new servers side by side), security (use the latest and safest base system at each deploy), and more.
However, throwing in a bunch of new servers at each one-line CSS change is going to be complicated, not to mention costly.
Containers to the rescue! Creating container "golden images" is easy, fast, dare I say painless. Replacing your old containers with new ones is also easy to do; much easier than virtual machines, let alone physical ones.
In this talk, we'll quickly recap the pros (and cons) of immutable servers; then explain how to implement that pattern with containers. We will use Docker as an example, but the technique can easily be adapted to Rocket or even plain LXC containers.