PR: ARG build-time parameter for docker 1.9

Hi,

I’ve just pushed a PR that implements a new Dockerfile extension in Docker 1.9 - ARG command.
The ansible module docker_image now supports this!

Someone please review, as soon as it’s said ok, I’m going to create another PR for the 2.0 version.

Alexy, I cant find your PR. Can you give me url?

Here you are https://github.com/ansible/ansible-modules-core/pull/2460