feat: update Docker configuration for combined frontend and backend build process

Dockerfile

@@ -0,0 +1,46 @@
+######## Root Combined Frontend + Backend Build ########
+## Build args (can be overridden)
+ARG NODE_VERSION=20-bookworm-slim
+
+########################
+# 1) Frontend build     #
+########################
+FROM node:${NODE_VERSION} AS frontend-build
+WORKDIR /frontend
+COPY frontend/package*.json ./
+RUN npm install --no-audit --no-fund
+COPY frontend/ .
+RUN npm run build
+
+########################
+# 2) Backend build      #
+########################
+FROM node:${NODE_VERSION} AS backend-build
+WORKDIR /app
+COPY backend/package*.json ./
+RUN npm install --no-audit --no-fund
+COPY backend/ .
+# Copy built frontend assets
+COPY --from=frontend-build /frontend/dist ./public
+RUN npm run build
+
+########################
+# 3) Production image   #
+########################
+FROM node:${NODE_VERSION} AS runner
+ENV NODE_ENV=production
+WORKDIR /app
+
+# Install production deps
+COPY backend/package*.json ./
+RUN npm install --omit=dev --no-audit --no-fund
+
+# Copy backend dist + public
+COPY --from=backend-build /app/dist ./dist
+COPY --from=backend-build /app/public ./public
+
+ENV PORT=3000 \
+    SERIAL_BAUD=115200
+
+EXPOSE 3000
+CMD ["node", "dist/main.js"]