Override root default superuser name

[metametadata]

Would be great to have something like MYSQL_ROOT_USER environment variable to change the default root superuser name.

E.g. PostgreSQL (https://hub.docker.com/_/postgres) has POSTGRES_USER for that.

[LaurentGoderre]

I can't find this usecase in the MySQL documentation. It's possible that it isn't meant to be configurable.