Change default MySQL/MariaDB charset + collation to utf8mb4

The current docker compose file for MYSQL is not installing with the correct character set and collation which is breaking the Notifications application per the below issue.

https://forum.xwiki.org/t/error-unknown-macro-notifications-under-my-active-stream-on-the-user-profile-page/11074

The issue goes away when adding the below line in to the Docker Compose file which changes the default character set to utf8mb4 and utf8mb4_bin.

command: ['mysqld', '--character-set-server=utf8mb4', '--collation-server=utf8mb4_bin'] 

Note: this may also be an issue for the mariadb compose setup but I have not tested it.

Thank you.