I noticed the other day that localtime within Mattermost App is UTC. Host ist running on CEST and so I had a difference of 2h. Checking inside the container I saw that /etc/localtime was pointing correctly to /usr/share/zoneinfo/Europe/Berlin but the file didn’t exist.
As a workaround I copied /usr/share/zoneinfo/Europe/Berlin from host into container and everything is fine now.
Someone should fix this in an updated container version. By the way, any chance that Mattermost App gets updated soon? Version in App Center (5.10) is rather old compared to current stable version (5.22).