Why did mysql data ownership change to systemd-journal-remote after running a docker container
You should start Docker's container with --user parameter. If you do this and set the same uid:gid as owner of the MySQL storage you will no have problems with permissions. You have to check how exactly to do this in Docker Compose because I show you example for normal command line execution
Most likely, uid of your user systemd-journal-remote is the same as uid of user mysqld in container. Check with ls -n. To avoid confusion, either use common uids, perhaps test as root:root with chmod o+rwx.