Description
We are using docker images for running tests where the downloaded composer-packages are cached between
the docker images to reduce download time. However, the composer packages for PHP-Unit are not cached
and downloaded every time which increases the start-time over 30 seconds.

How to reproduce

mkdir test && cd test
export COMPOSER_CACHE_DIR=composer_cache/
composer require symfony/phpunit-bridge
vendor/bin/simple-phpunit --help

rm -rf vendor
composer require symfony/phpunit-bridge
vendor/bin/simple-phpunit --help

Possible Solution
Cache vendor/bin/.phpunit, but this is the final installation directory.