Add .dockerignore file support

使用 Funfile 构建，如果项目中有大的临时数据目录，fun build 开始执行时会占用很高 CPU，执行很慢，移除大文件后速度飞快。

已有 .funignore 文件：

```
data/
.env
template.yml
```

项目中添加 .dockerignore 文件没有效果，内容和 .funignore 一样。

希望能支持 .dockerignore 文件，或者让 docker build 执行时使用 .funignore。

Start by tracing how `fun build` prepares its build context and how the Docker build is invoked, comparing the existing `.funignore` handling with `.dockerignore` files. Verify the behavior with a project containing a large `data/` directory and confirm that the selected ignore-file behavior prevents that directory from being sent to the build.

Written by the indexing model from the issue text.