fix: correct how Dockerfile copies dirs

frontend/Dockerfile

@@ -1,8 +1,14 @@
-FROM node:20-alpine AS builder
+# --- Build React app ---
+FROM node:22-alpine AS builder
 
 WORKDIR /app
-COPY . .
-RUN npm install && npm run build
 
+COPY frontend/package*.json ./
+RUN npm install
+
+COPY frontend/ ./
+RUN npm run build
+
+# --- Serve with Nginx ---
 FROM nginx:alpine
 COPY --from=builder /app/build /usr/share/nginx/html