Add Dockerfile with multi-stage build configuration

Dockerfile

@@ -0,0 +1,43 @@
+# Stage 1: Build Frontend
+FROM node:20-alpine AS frontend-builder
+
+WORKDIR /app/frontend
+
+COPY frontend/package*.json ./
+RUN npm ci
+
+COPY frontend/ ./
+RUN npm run build
+
+# Stage 2: Production Server
+FROM node:20-alpine
+
+WORKDIR /app
+
+# Install production dependencies
+COPY backend/package*.json ./
+RUN npm ci --only=production && \
+    npm cache clean --force
+
+# Copy backend source
+COPY backend/ ./
+
+# Copy built frontend
+COPY --from=frontend-builder /app/frontend/dist ./public
+
+# Create temp upload directory
+RUN mkdir -p /app/temp && \
+    chown -R node:node /app
+
+# Switch to non-root user
+USER node
+
+# Expose port
+EXPOSE 3000
+
+# Health check
+HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
+    CMD node -e "require('http').get('http://localhost:3000/api/health', (r) => {process.exit(r.statusCode === 200 ? 0 : 1)})"
+
+# Start server
+CMD ["node", "src/server.js"]