Docker image, & local development docker-compose.yml (#2)

Adds:
- a Docker image, which can be used in both development and production
- a `docker-compose.yml` file for local development (could probably be adapted for production deployments)

Testing:
- `git checkout docker`
- `docker-compose up`
- pray 🙏
- go to http://localhost:5000

## App architecture

I added the ability to load secret config variables (`HUB_TOKEN`, `STRIPE_SECRET_KEY` etc) from files, to support [Docker Secrets](https://docs.docker.com/engine/swarm/secrets/) natively. The code should probably log an error if a specified `HUB_TOKEN_FILE` doesn't exist instead of failing silently..

## Docker architecture

This uses a multi-stage build to reduce the size of the final image -- having pipenv install to a predefined virtualenv, and then copying that over.

The compose file doesn't include a definition for a cron runner service, and I haven't tested running one yet. Here be dragons!

You can rebuild the image locally using `docker-compose build`, but this isn't required for changes to the app code, only if you edit the `Dockerfile`, or want to publish your image for use on a swarm server (in which case you will need to edit the image name to put in your own Docker hub credentials).

Currently, the image is rebuilt (should set up auto-tagging..) and published with every push to this 3wordchant/capsul-flask fork.

Reviewed-on: 3wordchant/capsul-flask#2
Co-authored-by: 3wordchant <3wordchant@noreply.git.autonomic.zone>
Co-committed-by: 3wordchant <3wordchant@noreply.git.autonomic.zone>

Dockerfile

@@ -0,0 +1,48 @@
+FROM python:3.8-alpine as build
+
+RUN apk add --no-cache \
+    build-base \
+    gcc \
+    gettext \
+    git \
+    jpeg-dev \
+    libffi-dev \
+    libjpeg \
+    musl-dev \
+    postgresql-dev \
+    python3-dev \
+    zlib-dev
+
+RUN mkdir -p /app/{code,venv}
+WORKDIR /app/code
+COPY Pipfile Pipfile.lock /app/code/
+
+RUN python3 -m venv /app/venv
+RUN pip install pipenv setuptools
+ENV PATH="/app/venv/bin:$PATH" VIRTUAL_ENV="/app/venv"
+RUN pip install wheel cppy
+# Install dependencies into the virtual environment with Pipenv
+RUN pipenv install --deploy --verbose
+
+FROM python:3.8-alpine
+
+RUN apk add --no-cache \
+    cloud-utils \
+    libjpeg \
+    libpq \
+    libstdc++ \
+    libvirt-client \
+    openssh-client \
+    virt-install
+
+COPY . /app/code/
+WORKDIR /app/code
+
+COPY --from=build /app/venv /app/venv
+ENV PATH="/app/venv/bin:$PATH" VIRTUAL_ENV="/app/venv"
+
+CMD ["gunicorn", "--bind", "0.0.0.0:5000", "-k", "gevent", "--worker-connections", "1000", "app:app"]
+
+VOLUME /app/code
+
+EXPOSE 5000