Build/Test Tools: Expose port 3306 of MySQL container.

This only defines the container port to allow `docker-compose` to choose an ephemeral host port.

Props ocean90, peterwilsoncc.
Fixes #48281.
Built from https://develop.svn.wordpress.org/trunk@47225


git-svn-id: https://core.svn.wordpress.org/trunk@47025 1a063a9b-81f0-0310-95a4-ce76da25c4cd