Alpine Linux 명령어로 수정

docker/prod/backend.Dockerfile

@@ -23,8 +23,8 @@ RUN npm run build
 FROM base AS runner
 ENV NODE_ENV=production
 
-# Create non-root user
-RUN groupadd -r appgroup && useradd -r -g appgroup appuser
+# Create non-root user (Alpine 방식)
+RUN addgroup -S appgroup && adduser -S -G appgroup appuser
 
 # Copy production node_modules
 COPY --from=deps /app/node_modules ./node_modules