doc: mysql permissions error and mitigation

[chickenandpork]

Running into a deployment issue, I wanted to document the mitigation.

The TL;DR is: mitigate before docker-compose up using: chmod -R ugo+rx env/mysql

Of note: my docker-compose is a 1.28 whereas the most recent commit(s) have upgraded for a docker-compose 2.7. That may be the source of my issue, and if so, I'd prefer to still document it for others in case it's related to an evolving mariadb container.