Explicitly expose backend ports in Dockerfile

backend/Dockerfile

@@ -8,5 +8,7 @@ RUN npm run build
 FROM node:alpine
 WORKDIR /app
 COPY --from=builder /app ./
+
+EXPOSE 3000
 CMD ["npm", "run", "start:prod"]
 

backend/src/main.ts

@@ -3,6 +3,6 @@ import { AppModule } from './app.module';
 
 async function bootstrap() {
   const app = await NestFactory.create(AppModule);
-  await app.listen(3500);
+  await app.listen(3000);
 }
 bootstrap();