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

nginx/templates/default.conf.template

@@ -108,7 +108,7 @@ server {
       proxy_pass http://inv;
     }
 
-    root  /opt/inventaire/client;
+    root /opt/inventaire/client;
     location /public {
         include /etc/nginx/snippets/security_headers.conf;
         limit_except GET {