Change Bookstack to use linuxserver images and mariadb

This is an optional change @NickBusey, for consideration in moving to the images created by linuxserver instead.

I also changed the pinned versions to latest (Current is stuck at 5th May 2020 for no good reason)

Looking at linuxserver seems to have kept up the pace and handles migrations too.

NOTE: this migration worked for me, but I moved my own database instead of just renaming the docker image. So this would be fine on first install, but not for someone running the old instances!

So this warrants proper testing, specially if users want to make the same leap (didn't try to just switch the image and keep the same volumes - so don't know if mysql -> maria could work like that).

Alternatively could keep the mysql DB running? 🤷