I recently had an issue where my default umask on my OS removes other permissions.
Then the default docker mode, meant that the grafana container which is configured to use the grafana user couldn't load the bind mounted ~/.rocketpool/grafana-prometheus-datasource.yml:/etc/grafana/provisioning/datasources/prometheus.yml.
I had to do chmod o+r ~/.rocketpool/grafana-prometheus-datasource.yml so that the grafana container could run.
Given that all the other containers are already using root, I doubt there's much security increase with using a grafana user. Might as well default to root so that it can read the files by default.