This is inspired by the PHPUnit extension written that I contributed to. However, this offers design decisions based on how I felt it should function. vscode-docker-phpunit also will not be a drop-in replacement. This is intended to execute PHPUnit found within a Docker container.
Make sure you have PHPUnit (tested/used against 4.7.x) in the container. Set the phpunit.exec and phpunit.code_path values of the container. Also set the phpunit.docker_name config or use the supplied command.
From this point just run one of the various Docker-PHPUnit methods to run a test class/method/directory. A new output channel is created with the PHPUnit output along with some extension-specific info (container name and command executed).