🐛 Fix docker configuration for flower (#37)

{{cookiecutter.project_slug}}/docker-compose.shared.base-images.yml

@@ -4,3 +4,7 @@ services:
     image: postgres:11
   queue:
     image: rabbitmq:3
+    # Using the below image instead is required to enable the "Broker" tab in the flower UI:
+    # image: rabbitmq:3-management
+    #
+    # You also have to change the flower command as documented in docker-compose.shared.admin.yml