`devcontainer features test` should log install output

`devcontainer features test` is great for black box tests / end result tests of `install.sh`.
However I also want to make sure what the script does exactly, i.e. whitebox-test or interaction-test.

For example when `java` is already available, I want my script to not install `default-jre` as that's kind of stupid.
I cannot verify this via the end result, as it's always the same: `java` is available.

Of course I could write some file from my install script with such information, e.g. when a `debug` option is set to true.
However I don't want to have such an option in my feature?!
It does seem easier if the docker image as generated by `devcontainer features test` would `RUN` the script with `| tee install.log`?

Then it doesn't end up in the final feature and I could check the contents of install.log from `test.sh`.

`devcontainer features test` コマンドから始め、生成された Docker イメージが feature の `install.sh` をどのように実行するかを追跡します。feature の `test.sh` のフローを確認し、キャプチャされたインストール出力をどのように読み取れるかを判断します。また、最終的な feature に含めずにインストール出力をテストで利用できるようになった時点で作業完了とします。

索引モデルが issue の本文から書いたものです。