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.
Id like to see a proposal for what that might look like before we invest significant effort into it.  We should not however mistake the current code for a general purpose solution that captures all corner cases.  Curl and bash is NOT a production quality library when used in this was

> On Sep 24, 2014, at 4:29 PM, Maciej Szulik <notifications@github.com> wrote:
> 
> In images/builder/docker/docker-builder/build.sh:
> 
> > @@ -14,6 +14,17 @@ if [ -n "$DOCKER_REGISTRY" ]; then
> >    TAG=$DOCKER_REGISTRY/$BUILD_TAG
> >  fi
> >  
> > +if [[ $DOCKER_CONTEXT_URL != "git://"* ]] && [[ $DOCKER_CONTEXT_URL != "git@"* ]]; then
> > +  URL=$DOCKER_CONTEXT_URL
> > +  if [[ $URL != "http://"* ]] && [[ $URL != "https://"* ]]; then
> > +    URL="https://"$URL
> > +  fi
> > +  curl -sf $URL > /dev/null
> @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?
> 
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