Very few apps will ever get to the point where they have millions of users, and even fewer will need to use docker to orchestrate what should amount to < 5 servers (1 proxy, 2 web fronts, 1 cache, 1 db) to start to handle that kind of load.
Once you are at that kind of scale, where you are handling 5+ containers/machines, then it might make sense to look at docker, chef, puppet, etc... but for the rest of us, starting with docker feels like premature optimization for scale that will likely never happen.
The only sensible part of docker at small scale is if you are inside a larger org and you are deploying a lot of apps and you are using docker to simplify deployment (sort of like WAR files in javaland...sort of), but even then for php/mysql a git checkout or rsync works like a champ.