Forget my first instructions 
- Docker cp your latest backup out of the container. Backups live inside
/shared/backups/default. - Delete the shared folder from your installation directory (the place where you call ./launcher from):
rm -rf /var/discourse/shared - Rebuild the container using
./launcher rebuild <container-name>.
This should do the trick, but be careful not to lose your backup. I was trying to avoid to suggest it so you don’t accidentally delete your backup, but seems to be the only path forward.