Build parent images checking the Dockerfile directive

[anthony-o]

Fixes #1

[stephenmelrose]

Hey @anthony-o. Cheers for doing these 2 PRs. I won't properly get to look at these for a week or 2, just wanted to let you know you weren't being ignored.

Really like the idea of the parent images idea. Re: build errors, this is something that has annoyed me and I was going to fix it, but this is actually due to an error in docker itself. See https://github.com/docker/docker/issues/6977. I'd really like to avoid covering for issues with docker as the code will quickly get redundant once the issue is fixed. Thoughts @makeusabrew?

[makeusabrew]

Sorry for skirting these PRs guys, been really busy and not had any time to think them over. Trying to find a few hours this evening.

I've got no problem sniffing out errors; yes, it'd be nice to get a proper status code back from Docker but in the meantime we should be able to detect them and help the user out, as long as the code is annotated properly. I'm not 100% keen on how this is implemented in the PR though so if I can find the time I'll merge it locally and make a few tweaks first.