Add docker support for local unit testing

Addresses consistency across different OS's by making it very similar to
the base Travis image.

Dockerfile

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+FROM python:3.6
+
+ADD . /app
+WORKDIR /app
+
+RUN pip install tox

Makefile

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+test:
+	find . -name "*.pyc" -delete
+	docker build -t sanic/test-image .
+	docker run -t sanic/test-image tox