First of all, it is a little confusing that both of the following exist, so not entirely sure if we maintain the docs as well as the images as they are in different orgs.
The main README (docker-library) states that configuration of the MySQL instance should be done by overrides in the override dir:
$ docker run --name some-percona -v /my/custom:/etc/mysql/conf.d -e MYSQL_ROOT_PASSWORD=my-secret-pw -d percona:tag
Our docs suggest that this is not the case, e.g.
docker run --name my-new-container-name -v /my/custom/config-file:/etc/my.cnf -e MYSQL_ROOT_PASSWORD=my-secret-pw -d percona/percona-server:tag
The README that appears on the hub for percona/percona-server adds yet another different recommendation:
Using a Custom MySQL Config File
The MySQL startup configuration in these Docker images is specified in the file /var/lib/mysql/my.cnf. If you want to customize this configuration for your own purposes, you can make changes to this file.
At least for PS 8.0, you should probably be using /etc/my.cnf.d as it is configured for includes: