Summary:
Split out enable IPv6 on docker into its own script.
Do not do this on MacOS hosts; where we would not be running in a docker container.
use apt add-on rather; which automatically disabled for non-apt supporting OSes (MacOSX)
Pull Request resolved: https://github.com/facebook/folly/pull/1108

Reviewed By: vitaut

Differential Revision: D14899358

Pulled By: calebmarchent

fbshipit-source-id: 342e7a37e27317018d98d77e8603c896f34b3bc6