Add tmpfs for npm cache in read_only container

Allows Prisma CLI operations to work within the
security-hardened read_only container.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -26,6 +26,7 @@ services:
     read_only: true
     tmpfs:
       - /tmp
+      - /home/nextjs/.npm
 
   rtrips-postgres:
     image: postgres:16-alpine