change backend docker path

backend/Dockerfile

 FROM node:22-alpine
 WORKDIR /app
-# نسخ ملفات التعريف أولاً لتسريع البناء
 COPY package*.json ./
 RUN npm install --production
 COPY . .

captain-definition

 {
   "schemaVersion": 2,
-  "dockerfileLines": [
-    "FROM node:22-alpine",
-    "WORKDIR /app",
-    "COPY . .",
-    "RUN cd backend && npm install --production",
-    "EXPOSE 5000",
-    "CMD [\"sh\", \"-c\", \"cd backend && node src/app.js\"]"
-  ]
+  "dockerfilePath": "backend/Dockerfile"
 }
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