Take a backup:
root@discourse-docker-app:/# discourse backup
Starting backup...
…
[SUCCESS]
Backup done.
Output file is in: /var/www/discourse/public/backups/default/discourse-2020-01-20-163535-v20190603134013.tar.gz
back out to the host
copy the backup somewhere:
root@discourse-docker:~# cp /var/discourse/shared/standalone/backups/default/discourse-2020-01-20-163535-v20190603134013.tar.gz ~
start a rebuild:
root@discourse-docker:~# /var/discourse/launcher rebuild app