Well, usually /var/discourse/shared/standalone/backups is the directory that is visible in your container as /var/www/discourse/public/backups (hence the word shared). Your rsync command is putting the backup inside that directory in order to make it accessible from within the container.
Vice versa, if the container writes to public/backups it should be visible on your host in the shared directory.
I wrote /var/discourse/shared.... above. But it seems that you are working in ~/infra/discourse, so you are copying to ~/infra/discourse/shared/standalone/backups/default.
Usually the container is mapped to /var/discourse/shared/...
This might be the issue. Can you check if you have a /var/discourse/shared ?