I found the following thread from a user who had exactly the same problem and seems to have solved it:
He was also trying to locate where Mattermost is installed on the Qnap NAS and posts this as his solution:
Unfortunately he didn’t post the full path name so I ran the find command to find this directory.
[~] # sudo find / -type d -name "system-docker" /share/CACHEDEV1_DATA/.qpkg/container-station/var/run/system-docker /share/CACHEDEV1_DATA/.qpkg/container-station/run/system-docker /share/CACHEDEV1_DATA/.qpkg/container-station/system-docker
It found multiple directories with that name and none seems to include a directory named “mattermost”. I start to think that Mattermost is not installed in a docker container on my NAS but what do I know?
I am stuck again but maybe with a little help I can get closer to finding the install directory of Mattermost.