Docker-compose: Exposing /mattermost as volume leads to strange behaviour

Hi all, I am running Mattermost with docker-compose and would like to expose the full container /mattermost folder as a volume instead of the many individual ones (config, data, etc.)
To do so I copied the content of /mattermost to a local folder, renamed, set access rights, etc. but experience some strange behaviour afterwards.

Steps to reproduce

1. docker cp mattermost-app:/mattermost mattermost.full # Copy the content of the container mattermost folder to a local folder
2. docker-compose down --rmi all
3. mv mattermost  # Move the container folder content in place of the original volumes
4. mv mattermost.full mattermost # Move the container folder content in place of the original volumes
5. chown 2000:2000 -R mattermost # Make sure the owner matches
6. Change docker-compose.yml volume definition
      - ./mattermost:/mattermost      
      #- ./mattermost/config:/mattermost/config:rw
      #- ./mattermost/data:/mattermost/data:rw
7. docker-compose up -d

Expected behavior
Things work as before

Observed behavior
The system comes up fine at first glance. But for some reason posts with images won’t show the images any longer, only text, even though the data folder is available and identical to the one in the original folder. I also get a notice from the Mattermost advisor telling me that I need to set my email details, and indeed when checking in System Console there are no SMTP settings. But when checking config.json the settings are there. Other settings like the db connection string were applied fine.
I would have expected that switching volumes the way I did is completely transparent to Mattermost but it looks like something is going on which I don’t understand.

Anyone got an idea?