Dockerized tests

[avishayp]

In this PR:

• Move docker base image to python3-alpine instead of ubuntu.
• Reduce docker context size with more aggressive .dockerignore
• Add a service that runs test_httpbin.py to docker-compose.yml

why

I was not able to run tox tests on my mac. I did not dig into that - docker provides more reproducible environment for running unittests.

[javabrett]

Disclaimer: I'm not a collaborator on this project and don't have any say on what gets merged.

Consider decomposing this PR - the main thrust, which is to run tests, should be independent of the Docker base/build changes.

The tests should be run as part of the Docker build, not as a Compose service, e.g.:

diff --git a/Dockerfile b/Dockerfile
index 6e391a6..6b4da16 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -11,6 +11,7 @@ EXPOSE 80
 
 ADD . /httpbin
 
-RUN pip3 install --no-cache-dir gunicorn /httpbin
+RUN pip3 install --no-cache-dir gunicorn /httpbin && \
+    python3 /httpbin/test_httpbin.py

... or perhaps better, arrange for the equivalent of python3.6 setup.py test as mentioned in #448.

[kennethreitz]

let's break this out, keep the image the same for now

[avishayp]

Replaced with #520 as requested.