The Guardian has hundreds of servers running, pretty much all EC2 instances. EC2 images are baked and derived from official images, similarly to the way you bake a docker image.
We built tools before docker became the de facto standard, so we could easily keep the EC2 images up to date. We integrated pretty well with AWS so that the basic constructs of autoscaling and load balancer were well understood by everyone.
The stack is mostly JVM based so the benefits of running docker locally weren't really significant. We've evaluated moving to a docker solution a few times and always reached the conclusion that the cost of doing so wouldn't be worth the benefits.
Now for a company that starts today I don't think I'd recommend that, it just so happen that The Guardian invested early on the right tooling so that's pretty much an exception.