`.dockerignore`: Use an allowlist approach for selective builds

For very selective builds, a robust strategy is to ignore everything by default (*) and then use the negation pattern (!) to explicitly include only the necessary files or directories.

Example:

```
# Ignore everything
*
# Include only what's needed
!src/
!package.json
!package-lock.json
```

Seems it has not been updated for 8 years:

https://github.com/apache/cloudstack/blob/main/.dockerignore

从仓库根目录中的 .dockerignore 开始，issue 指出该文件已过时，并将其当前模式与提议的 allowlist 示例进行比较。验证所得的排除项仍允许 issue 中描述的选择性构建和所需路径。

由索引模型根据 Issue 内容生成。