Add support for chromium which now finally supports arm64

[skodak]

Hi, chromium selenium now supports arm64 (and Apple MX processors) and seleniarm is dead. please note selenium/standalone-chrome does not support arm64 yet. I have tried to add support for it to my fork, I faced a problem with the configuration in config.php, so I thought I might save you some time by giving you link to my changes

https://github.com/skodak/moodle-docker/commit/3f144d55d68ef4e2eb7db752b44b6177a7929c09

Ciao