Use named volume for notebooks in production

Bind mounts inherit host UID which conflicts with the container's
non-root user. Named volumes are initialized from the image layer,
preserving the chown from the Dockerfile.

docker-compose.prod.yml

@@ -4,6 +4,8 @@ services:
       context: ./backend
       dockerfile: Dockerfile
       target: prod
+    volumes:
+      - notebooks:/app/notebooks
     expose:
       - "8000"
     networks:
@@ -28,6 +30,9 @@ services:
       caddy: "${SPICEBOOK_DOMAIN:-spicebook.localhost}"
       caddy.reverse_proxy_1: "{{upstreams 4321}}"
 
+volumes:
+  notebooks:
+
 networks:
   caddy:
     external: true