Issue with volume permissions between containers / not understanding them correctly?

I’m a bit confused as to how permissions work when mounting a volume

I have two containers mariadb and fluentd

both have access to the hosts /log/mariadb folder

mariadb writes a file called my.log to that folder and on the host it shows up with the user:group of systemd-coredump:systemd-coredump

so my host user myuser dosnt have access to that group so I added the group systemd-coredump to myuser

now I can view the file fine from the host

the problem comes when I try and read the file from inside my fluentd container

I thought my flientd container would be running as myuser and would have access to the file the same way the host does?

does docker or the system need restarted for the new group permissions to be picked up or did I misunderstand how this all works?