Docker is great also to hide complexity during implementation. Discourse.org is doing a great work "enveloping" their complex rails app in containers to easy the install process. And is not stateless.
You mean hide ugly sprawling uninstallable messes of rube goldberg code? :)
I'm referring to things that require PHP+MySQL, Node, Redis, and an old JVM process running Struts/Spring all managed by nginx except for that one situation where Apache2 .htaccess semantics are required for rewrite rules in which case it runs Apache proxied by nginx.
A great example is the Chef server rpm. It is a 500mb mini distribution in one package. It has copies of perl, python, Ruby, and Erlang in it. If any of these has a security vulnerability, I have to wait on the maintainer to release a new version, and hope it included the security fixes.
They also tend to include things like python header files for no reason. You wouldn't compile against an Omnibus package, but they are there anyway. Examples of this are Sumologic's and Datadog's agents.
The result is that their forum software requires another operating system to run. Had they been more disciplined in their development approach, Docker would have been merely a convenient way to test Discourse, and not the only supported option.