phpunit is not available inside the container

In the documentation, there is a mention of a test command to run PHPUnit. The container does not have PHPUnit installed.

Perhaps we can spin up a separate PHPUnit container in the development environment.

Start with the documentation's PHPUnit test command and inspect the container setup for the development environment. Determine how PHPUnit should be made available, then verify that the documented command runs successfully inside the container.

Written by the indexing model from the issue text.