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>

capsulflask/__init__.py

@@ -27,8 +27,24 @@ class StdoutMockFlaskMail:
     def send(self, message: Message):
       current_app.logger.info(f"Email would have been sent if configured:\n\nto: {','.join(message.recipients)}\nsubject: {message.subject}\nbody:\n\n{message.body}\n\n")
 
+
 load_dotenv(find_dotenv())
 
+for var_name in [
+  "SPOKE_HOST_TOKEN", "HUB_TOKEN", "STRIPE_SECRET_KEY",
+  "BTCPAY_PRIVATE_KEY", "MAIL_PASSWORD"
+]:
+  var = os.environ.get(f"{var_name}_FILE")
+  if not var:
+    continue
+
+  if not os.path.isfile(var):
+    continue
+
+  with open(var) as secret_file:
+    os.environ[var_name] = secret_file.read().rstrip('\n')
+  del os.environ[f"{var_name}_FILE"]
+
 app = Flask(__name__)
 
 app.config.from_mapping(