fix: use root build context in docker-compose.prod.yml

Dockerfiles reference monorepo subdirectories (frontend/, backend/)
but the prod compose specified ./frontend and ./backend as build
contexts. The containers were running because they were built from
docker-compose.yml (dev) which already had root context. Rebuilds
with --build were broken.

docker-compose.prod.yml

@@ -66,8 +66,8 @@ services:
   # ==========================================
   backend:
     build:
-      context: ./backend
+      context: .
-      dockerfile: Dockerfile
+      dockerfile: ./backend/Dockerfile
     container_name: coop-backend
     environment:
       - NODE_ENV=production
@@ -126,8 +126,8 @@ services:
   # ==========================================
   frontend:
     build:
-      context: ./frontend
+      context: .
-      dockerfile: Dockerfile
+      dockerfile: ./frontend/Dockerfile
       args:
         - NEXT_PUBLIC_API_URL=https://coop.hobokenchicken.com
         - NEXT_PUBLIC_SOLANA_NETWORK=devnet