Discovered today that Vagrant automatically rsyncs files to the Host VM when the Docker provider is used on a system that doesn’t natively support Docker.
Sometimes, though, one wants to sync the files to a custom location, and might get errors when trying to sync them to two locations on the Host VM (as I was).
To turn off Vagrant’s default sync, use the same syntax you would to turn off the /vagrant shared folder otherwise:
config.vm.synced_folder ".", "/vagrant", disabled: true
Thanks to @adamjt on GitHub for the tip: https://github.com/mitchellh/vagrant/issues/3680#issuecomment-42482434