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>

docker-compose.yml

@@ -0,0 +1,36 @@
+---
+version: "3.8"
+
+services:
+  app:
+    image: 3wordchant/capsul-flask:latest
+    build: .
+    volumes:
+      - "./:/app/code"
+      - "../tank:/tank"
+      # - "/var/run/libvirt/libvirt-sock:/var/run/libvirt/libvirt-sock"
+    depends_on:
+      - db
+    ports:
+      - "5000:5000"
+    environment:
+      - "POSTGRES_CONNECTION_PARAMETERS=host=db port=5432 user=capsul password=capsul dbname=capsul"
+      - SPOKE_MODEL=shell-scripts
+        #- FLASK_DEBUG=1
+      - BASE_URL=http://localhost:5000
+      - ADMIN_PANEL_ALLOW_EMAIL_ADDRESSES=3wc.capsul@doesthisthing.work
+      - VIRSH_DEFAULT_CONNECT_URI=qemu:///system
+    # The image uses gunicorn by default, let's override it with Flask's
+    # built-in development server
+    command: ["flask", "run", "-h", "0.0.0.0", "-p", "5000"]
+  db:
+    image: "postgres:9.6.5-alpine"
+    volumes:
+      - "postgres:/var/lib/postgresql/data"
+    environment:
+      POSTGRES_USER: capsul
+      POSTGRES_PASSWORD: capsul
+      POSTGRES_DB: capsul
+
+volumes:
+  postgres: