Add HTTP(S) proxy support in the build process

[trexmaster]

I had to manually edit the Dockerfiles to add the environment variables to use the HTTP proxies, it'd be nice if the Makefile could just get them from the running environment and pass them to the rest of the components.

[hbbio]

Can you submit a pull request?

[trexmaster]

I'm not used to git so I'll answer with maybe :) I've forked the project, so I'll try to work there and when I'm confident enough I can make a pull request without breaking stuff I'll do it.

As a side note, I just found out that Docker is in the process of adding build-time environment variables (see https://github.com/docker/docker/pull/9176) which would probably solve this issue, or at least help a great deal.

Edit : Ah ! Turns out forking's exactly what I was supposed to do :D

[hbbio]

Yes, a few new Docker features would definitely help! Also, we reviewed wether the added complexity of running etcd in containers would be a better replacement for all configuration currently in /etc/peps but decided to wait until things stabilize a bit on the Docker side.

And we will review pull requests anyway.