Fail fast when non-existent URL passed to docker build

This PR is to address issue talked in #115 
@smarterclayton @csrwng @mfojtik lemme know what do you think?

The other thing is, I'd like to extend our integration tests a little bit, as it was talked some time ago on IRC with possibility to test e.g. this as well. I'll working on it as well, unless told otherwise.
@ironcladlou the problem here was that when git was accessing non-existent repository it though that it might be private (priv repos are 404-ed on github) so it asked for credentials. Plain `curl` will get 404 which is what I wanted to achieve with it. I've added `--max-time` and `--location` as @mfojtik suggested. I don't think we need to address wrong cert issue here, as git will have exactly the same problems accessing those.