Add Dockerfile so everyone can get the same results

[talex5]

e.g. formatting of the code blocks in /faq/ doesn't work correctly with Debian's version of mkdocs.

[bastianeicher]

How about the following Dockerfile instead:

FROM python:3.7-slim
COPY requirements.txt .
RUN pip install -r requirements.txt
WORKDIR /app
ENV LC_ALL=C.UTF-8 LANG=C.UTF-8
ENTRYPOINT ["mkdocs"]

This would ensure that Mkdocs version in Docker stays in sync with the one used in Travis CI.

[talex5]

OK - updated!