Compare with docker option

[Ari Maniatis]

The readme might do with some pros and cons compared the docker ways of doing this. Some thoughts:

• speed (my tests show this one slightly slower at around 40 seconds compared to 25)
• client tools. This approach installs the proper mariadb client executable accessible to the tests
• avoid collisions. This approach uninstalls mysql which is already installed in github actions.
• socket connections available (haven't tested yet as to whether that will help with speed or reliability)

[Mateusz Łoskot]

IMHO, this approach is a very good option, if not the only, for windows-based jobs.