Add Dockerfile and docker-compose.yml to run tests and linter

Just run `docker-compose up test` and `docker-compose up lint`
to run tests and the linter without setting up a local python environment.

Dockerfile

@@ -0,0 +1,7 @@
+# Dockerfile to run the test suite and linter
+FROM python:slim
+RUN pip install flake8 flake8-import-order doc8 yamllint \
+ && apt-get update \
+ && apt-get install -y locales \
+ && rm -rf /var/lib/apt/lists/* \
+ && localedef -i en_US -c -f UTF-8 -A /usr/share/locale/locale.alias en_US.UTF-8