Iterate local development with Docker

This makes a few changes to:

- Make local development consistent with our other apps. It's now
faster to start Celery locally since we don't try to build the
image each time - this is usually quick, but unnecessary.

- Add support for connecting to a local Redis instance. Note that
the previous suggestion of "REDIS = True" was incorrect as this
would be turned into the literal string "True".

I've also co-located and extended the recipes in the Makefile to
make them a bit more visible.

app/config.py

@@ -428,7 +428,7 @@ class Development(Config):
     NOTIFY_EMAIL_DOMAIN = "notify.tools"
 
     SQLALCHEMY_DATABASE_URI = os.getenv('SQLALCHEMY_DATABASE_URI', 'postgresql://localhost/notification_api')
-    REDIS_URL = 'redis://localhost:6379/0'
+    REDIS_URL = os.getenv('REDIS_URL', 'redis://localhost:6379/0')
 
     ANTIVIRUS_ENABLED = os.getenv('ANTIVIRUS_ENABLED') == '1'