Comment 3 for bug 1879598

The issue here appears to be related to the docker image used for mattermost and a limitation of how juju is allowed to exec into the mattermost container.

There's this k8s bug

https://github.com/kubernetes/kubernetes/issues/30656

The mattermost docker image used by he charm sets up a non-root default user. However, when juju execs into the container as a result of upgrade charm, in order to unpack the new charm so actions are updated, the non-root user does not have permission to access the charm files, hence the permission error.

The only solution at the moment is to not have a default non-root user in the workload container.