replace shared volume between the inventaire and nginx services by a multi-stage copy

See https://docs.docker.com/build/building/multi-stage/#use-an-external-image-as-a-stage

The shared volume was making updates complicated, as the persisted volume data would not be overriden by the new container

docker-compose.yml

@@ -10,7 +10,6 @@ services:
       - couchdb
       - elasticsearch
     volumes:
-      - inventaire-server:/opt/inventaire
       # The following paths are outside /opt/inventaire so that the inventaire-server volume can be removed
       # (as required when updating the server and client) without loosing their data
       - inventaire-leveldb:/opt/inventaire_leveldb
@@ -68,7 +67,6 @@ services:
     volumes:
       - ./nginx/templates:/etc/nginx/templates/
       - ./nginx/snippets:/etc/nginx/snippets
-      - inventaire-server:/opt/inventaire
       - certbot-www:/var/www/html
       - ./certbot/conf:/etc/letsencrypt
       - nginx-cache:/tmp/nginx