I did this procedure, and the rebuild went fine (and ran all the updates as promised - 
!).
BUT my data is still there. Deleting shared didn’t delete my data. Judging by the results of du I think my data is in standalone subdir, not in shared subdir…
root@discourse:/docker-app/discourse# du -h /docker-app/ --max-depth=1
2.6M /docker-app/discourse
1.3G /docker-app/standalone
1.3G /docker-app/
So, can I delete docker-app/standalone? And did I delete anything important that I shouldn’t have in docker-app/discourse/shared?
P.S. - I did a snapshot of this VM before the delete, so I can go back in time if necessary