Mounting files to be used during features build

I'd like to be able to mount some files which then be later used during feature installation.

My use case looks as follows:
1. Before running `devcontainer up` I download some files
2. Then custom feature needs to do some processing on downloaded files - but it fails since those files are not available during build

I can see that within the BuildKit there is an option that we can leverage:
https://docs.docker.com/reference/dockerfile/#run---mount

从 `devcontainer up` 流程和 feature 安装路径入手，然后将所请求的行为与 BuildKit 的 `RUN --mount` 选项进行比较。当启动前下载的文件能够在 feature 安装期间使用，且不会破坏现有的 dev container 构建时，即可视为完成。

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