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 postmanlabs#493.

Author: javabrett

Dockerfile

@@ -5,12 +5,18 @@ LABEL version="0.9.2"
 LABEL description="A simple HTTP service."
 LABEL org.kennethreitz.vendor="Kenneth Reitz"
 
-RUN apt update -y && apt install python3-pip -y
+ENV LC_ALL=C.UTF-8
+ENV LANG=C.UTF-8
 
-EXPOSE 80
+RUN apt update -y && apt install python3-pip git -y && pip3 install --no-cache-dir pipenv
+
+ADD Pipfile Pipfile.lock /httpbin/
+WORKDIR /httpbin
+RUN /bin/bash -c "pip3 install --no-cache-dir -r <(pipenv lock -r)"
 
 ADD . /httpbin
+RUN pip3 install --no-cache-dir /httpbin
 
-RUN pip3 install --no-cache-dir gunicorn /httpbin
+EXPOSE 80
 
 CMD ["gunicorn", "-b", "0.0.0.0:80", "httpbin:app", "-k", "gevent"]