@heifner Sorry for the second PR for the same thing. I royally screwed up the previous branch and figured I'd just start off fresh to be safe.

Is setting the proxy in yum's config file not workable?

@benbenben1010 could answer that, but I personally believe having to add the proxy settings to each and every distro (and remember they're set there & it's not just YUM that needs this) would be a lot more work.

The -E is nice because it allows us to use the same Dockerfile in the sandbox (which requires the proxy) and outside the sandbox (which doesn't) by conditionally passing in environment variables to the docker build command.

That being said, building in the sandbox has been de-prioritized, so for now, I don't have a strong opinion either way.