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: #2
Co-authored-by: 3wordchant <3wordchant@noreply.git.autonomic.zone>
Co-committed-by: 3wordchant <3wordchant@noreply.git.autonomic.zone>

.drone.yml

@@ -0,0 +1,13 @@
+---
+kind: pipeline
+name: publish docker image
+steps:
+  - name: build and publish
+    image: plugins/docker
+    settings:
+      username:
+        from_secret: docker_reg_username_3wc
+      password:
+        from_secret: docker_reg_passwd_3wc
+      repo: 3wordchant/capsul-flask
+      tags: ${DRONE_COMMIT_BRANCH}