Docker containers for running tests is now available

[mringler]

I use Docker containers to run the test suite (and linters, checkers, etc), the configuration is now available at perplorm/perpl-test-docker.

If you have docker installed, I find this the most convenient way to run the project. All requirements are taken care of without installing anything into the OS, and you can switch DBMS or PHP version easily. XDebug and opening files from command line output works too. The custom terminal prompt makes it easy to know where you are.

The setup comes from Linux with rootless Docker (runs without admin privileges), particularly setting IP address for XDebug might differ on other setups (as in "is less annoying"). Hints to what should be added to the README are very welcome, as all other reports and feedback.