build: Add cache mounts to Dockerfile for faster Go binary compilation.

Dockerfile

@@ -51,8 +51,10 @@ COPY backend/ ./
 # Copy frontend dist from previous stage
 COPY --from=frontend-builder /app/frontend/dist ./internal/web/dist
 
-# Build the binary with embed tag
-RUN GOTOOLCHAIN=auto CGO_ENABLED=1 GOOS=linux go build \
+# Build the binary with embed tag (使用缓存加速)
+RUN --mount=type=cache,target=/root/.cache/go-build \
+    --mount=type=cache,target=/go/pkg/mod \
+    GOTOOLCHAIN=auto CGO_ENABLED=1 GOOS=linux go build \
     -tags embed \
     -ldflags="-s -w" \
     -o /app/codex-pool \