`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`.