I've been involved in that ticket since the dark ages. There's a philosophical disconnect between how (some) people want to use Docker and how the maintainers want it to be used, and it comes to a head in that thread.
I saw your articulate response in that thread. Thank you for that response. I don't need multiple dockerfiles right now, but if i invest more into docker, I will. I think your responses might have convinced shykes that the need won't be going away.
There was a similar disconnect on data volumes a while ago that took some convincing before the discussion moved forward. It has always been trivial in OpenVZ to bind mount into a container (ie: share a very large read-only mount between containers).
Wow. That issue has been open for a long time.