Fix Dockerfile syntax

Dockerfile.backend

@@ -1,8 +1,10 @@
 FROM node:18-alpine
 
-WORKDIR /app/backend
+WORKDIR /app
 
-COPY backend/package*.json ./
+COPY backend/package*.json ./backend/
+
+WORKDIR /app/backend
 
 RUN npm install --production
 

Dockerfile.frontend

@@ -1,8 +1,10 @@
 FROM node:18-alpine
 
-WORKDIR /app/frontend
+WORKDIR /app
 
-COPY frontend/package*.json ./
+COPY frontend/package*.json ./frontend/
+
+WORKDIR /app/frontend
 
 RUN npm install