Build/Test Tools: Configure Xdebug modes in the local Docker environment.
One change in the update from Xdebug version 2.x to 3.x was a shift from enabling features to switching into modes.
When the version of Xdebug installed in the PHP 7.4 Docker container was updated from 2.x to 3.x, the code coverage reporting workflow stopped generating reports due to a lack of available coverage drivers.
This change adds the XDEBUG_MODE environment variable to the local Docker environment configuration to allow the active modes to be changed. This environment variable takes precedence over the xdebug.mode setting, but will not change the value of the xdebug.mode setting.
The LOCAL_PHP_XDEBUG_MODE environment variable has been added to the .env file and can be used to change the modes enabled in the Docker container. The code coverage reporting workflow uses this variable to enable the coverage mode, which is required for generating a test coverage report.
By default, debug and `debug modes are active, which enables the more commonly used features of Xdebug: development helpers and step debugging.
Props afragen, johnbillion, desrosj.
Fixes #56022.