Hey former Guardian employee here. 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…
This is an underappreciated point I think sometimes. Once you have a team which is familiar with your current, working setup, the benefits of moving away have to be pretty huge for it to be worthwhile.