ci(workflow): 修复docker.yml中格式错误并添加Flutter配置

修复脚本中的格式错误，确保正确换行。同时添加Flutter配置步骤以支持Windows桌面开发

.gitea/workflows/docker.yml

@@ -160,7 +160,8 @@ jobs:
         Write-Host "📄 验证文件："
         if (Test-Path libcore/bin) {
           Get-ChildItem libcore/bin -ErrorAction SilentlyContinue | Format-Table Name, Length
-        }\n\n    - name: Setup Flutter\n      uses: subosito/flutter-action@v2\n      with:\n        flutter-version: '3.24.5'\n        channel: 'stable'
+        }
+        \n\n    - name: Setup Flutter\n      uses: subosito/flutter-action@v2\n      with:\n        flutter-version: '3.24.5'\n        channel: 'stable'
         
         - name: Enable Windows desktop
           run: flutter config --enable-windows-desktop