Good write up, but Docker's also useful beyond having the ability to scale, portability being one. With some websites having a huge variety of dependencies now, keeping that all together and knowing it will run on multiple hosts is a big benefit.

(Downside, because nothing is ever for free: You then have to maintain those dependencies and update the image, rather than letting the OS keep everything up to date)

We use docker a lot at work, but not for scaling and not with any cloud provider.