Changed Dockerfile to resolve deps using pipenv/Pipefile(.lock) before pip3->setup.py for httpbin.
Changed Dockerfile to resolve deps using pipenv/Pipefile(.lock) before pip3->setup.py for httpbin.

Also added Pipfile(.lock) prior to the remaining source, improving Docker image cacheability.

Git is also required to fetch pyyaml.

This results in more deterministic and reproducible image builds, since httpbin dependencies
are installed using locked versions from Pipfile.lock before httpbin is itself installed.

Fixed #493.

kennethreitz/httpbin,postmanlabs/httpbin,Runscope/httpbin,ewdurbin/httpbin,postmanlabs/httpbin,kennethreitz/httpbin,krissman/httpbin,Runscope/httpbin,krissman/httpbin,ewdurbin/httpbin