docker-php-extension-installer: Cleanup does not leave already installed packages

As far as I can see cleanup (run by default since f164c691cdafd0b10242a4bb251ed6b96e59f2d8) does not leave already installed packages, but please correct me if I’m wrong. This means that the following will not work

RUN apt-get install -y --no-install-recommends \
    a-package-i-need-later-on-and-is-needed-by-an-extension
RUN install-php-extensions an-extension
RUN something-that-needs-a-package-i-need-later-on-and-is-needed-by-an-extension

Shouldn’t just the delta of what was on the system and what is later on on the system be cleaned?