[WIP] Fix switching to 3.0 from 2.5

[adriendupuis]

• 3.0 → 2.5: [WIP] Fix switching to 2.5 from 3.0 #17
• 2.5 → 3.0: [WIP] Fix switching to 3.0 from 2.5 #18

[adriendupuis]

When container mariadb already exists when passing from 2.5 to 3.0, the log is full of
[Warning] InnoDB: Table mysql/innodb_index_stats has length mismatch in the column name table_name. Please run mysql_upgrade

• Removing the container before up --build, of course, works.
• Keeping the container and running docker-compose exec mariadb mysql_upgrade -proot; when is up also works.