I meant: for now I recreated a brand new vagrant machine, no copying of any previous backups. Did the bootstrapping and starting the docker container. Did the backup.
Nothing is seen in the vagrant machine outside of the docker machine.
I think I’ve found the issue: I’ve mounted this docker-shared folder in the enclosing vagrant machine.
config.vm.synced_folder "discourse/", "/var/discourse"
If I comment this out from my Vagrantfile, the backups “magically” appear.
So the problem was that Vagrant shared folder (to “up”) and docker shared folder (to “down”) compete and make it invalid.