Do not inject ARG on newer Docker versions.

[Cryptophobia]

From @miracle2k on May 9, 2017 7:9

https://github.com/deis/dockerbuilder/pull/118/files/13b2463d5311da00ac5125534ee7add19a89fa3a#r108909997

I think it might be a good idea to not inject the ARGs in newer Docker versions. For me, each ARG statement takes about 2 seconds to run (Running on Azure). With a bunch of config vars defined, this extends the build time by minutes for me.

As an aside I do wonder why the ARG takes so long when running on my Deis install. Building the Dockerfile directly is super fast. Is this a disk-bound operation?

Copied from original issue: deis/dockerbuilder#123

[Cryptophobia]

From @bacongobbler on May 9, 2017 14:15

In the next release we made it an opt-in approach so you shouldn't be seeing it used by default on v2.14.0.

[Cryptophobia]

From @miracle2k on May 10, 2017 7:23

I opted in, but I only need maybe two of the 15+ config args in the Docker Build 😢 .