Thanks @pfaffman for adding this, really painless now! (except for restoring a large backup which is always a little bit of a tense and long wait whatever the setup).
btw I think this path is incorrect: should be web-only
(by default) I think?:
volumes:
- volume:
host: /var/discourse/shared/web-only
guest: /shared
- volume:
host: /var/discourse/shared/web-only/log/var-log
guest: /var/log
PS I’ve edited the OP accordingly