fix: docker build - use root context for monorepo workspaces

- Change build context from ./frontend|backend to root (.)
- Update Dockerfiles to copy from correct paths
- Use npm install instead of npm ci (workspaces share root lockfile)
- Remove obsolete version attribute from docker-compose.yml

Fixes npm ci errors when package-lock.json is at workspace root.

docker-compose.yml

@@ -1,5 +1,3 @@
-version: '3.8'
-
 services:
   postgres:
     image: postgres:16-alpine
@@ -37,8 +35,8 @@ services:
 
   backend:
     build:
-      context: ./backend
-      dockerfile: Dockerfile
+      context: .
+      dockerfile: backend/Dockerfile
     container_name: coop-backend
     environment:
       - NODE_ENV=production
@@ -69,8 +67,8 @@ services:
 
   frontend:
     build:
-      context: ./frontend
-      dockerfile: Dockerfile
+      context: .
+      dockerfile: frontend/Dockerfile
     container_name: coop-frontend
     environment:
       - NEXT_PUBLIC_API_URL=http://localhost:3001