better installation instructions + fix missing volume in docker-compose

docker-compose.override.yml

@@ -76,8 +76,8 @@ services:
       context: ./nginx
       dockerfile: ./Dockerfile
     image: registry.gitlab.com/champs-libres/geonature-citizen/nginx
-    # volumes:
-    #  - ./nginx/default.conf:/etc/nginx/conf.d/default.conf
+    volumes:
+     - ./nginx/default.conf:/etc/nginx/conf.d/default.conf
     depends_on:
       - "media"
       - "citizen-front"