serve /user-media/ via nginx.

This change landed in
27fbed27e0
and is primarily used by our future ui-tests to serve XPI files.

docker-compose.yml

@@ -33,6 +33,7 @@ services:
     volumes:
       - ./static:/srv/static
       - ./site-static:/srv/site-static
+      - ./user-media/:/srv/user-media
 
   memcached:
     image: memcached:1.4