Improve running redis locally

- adds simple explicit instructions for how to install and run
  redis
- gets celery running in docker able to talk to redis running
  natively on your machine as per
  https://docs.docker.com/desktop/mac/networking/#i-want-to-connect-from-a-container-to-a-service-on-the-host

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'