Docker: added little list of Redis database numbers we're using

docker/docker-compose.yml

@@ -15,6 +15,10 @@ services:
         max-size:            "200k"
         max-file:            "20"
 
+  # Databases in use:
+  # 0: Flask Cache
+  # 1: Celery (backend)
+  # 2: Celery (broker)
   redis:
     image:                   redis:3.2.8
     container_name:          redis