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.
@smarterclayton I know what you mean, I was thinking the same way when I was writing those bytes of code. But on the other hand if docker w/ git does not give us that flexibility, we need to do it by ourselves. Should I take your proposal as a challenge to create such a thing?