Standardize pip cache strategy: Use BuildKit cache mount for both stage and prod

- Both Dockerfile.stage and Dockerfile.prod now use --mount=type=cache
- Faster builds with BuildKit cache mount (temporary, per-build)
- Requires: docker buildx or DOCKER_BUILDKIT=1

backend/Dockerfile.prod

@@ -9,8 +9,9 @@ WORKDIR /app
 # Copy requirements.txt
 COPY requirements.txt .
 
-# Install dependencies to a local directory (with cache for speed)
-RUN pip install --user -r requirements.txt
+# Install dependencies to a local directory (with cache mount for speed)
+RUN --mount=type=cache,target=/root/.cache/pip \
+    pip install --user -r requirements.txt
 
 # ============================================================
 # Final stage - Production image (tối giản)

backend/Dockerfile.stage

@@ -15,7 +15,7 @@ ENV ENV=development
 # Copy requirements.txt trước để tận dụng Docker cache
 COPY requirements.txt .
 
-# Cài đặt thư viện Python (Sử dụng cache mount để siêu nhanh)
+# Cài đặt thư viện Python (với cache mount)
 RUN --mount=type=cache,target=/root/.cache/pip \
     pip install -r requirements.txt