title: Vagrant with Docker Provider requires RSync Password
body: <p>Starting vagrant (up) with a docker provider creates an rsync issue because rsync  tries syncing with a docker user and asks me for its password, which of course I do not have. If I exchange the docker provider with a vm.box everything is fine.
  The messages I get are:</p>
  
  <pre><code>WDFM32388067A:vagranttest d022051$ vagrant up
  Bringing machine 'default' up with 'docker' provider...
  ==&gt; default: Docker host is required. One will be created if necessary...
      default: Docker host VM is already ready.
  ==&gt; default: Syncing folders to the host VM...
      default: The machine you're rsyncing folders to is configured to use
      default: password-based authentication. Vagrant can't script rsync to automatically
      default: enter this password, so you'll likely be prompted for a password
      default: shortly.
      default: 
      default: If you don't want to have to do this, please enable automatic
      default: key insertion using `config.ssh.insert_key`.
      default: Rsyncing folder: /Users/d022051/tmp/vagranttest/ =&gt; /var/lib/docker/docker_1437748320_29948
  docker@127.0.0.1's password: 
  There was an error when attempting to rsync a synced folder.
  Please inspect the error message below for more info.
  
  Host path: /Users/d022051/tmp/vagranttest/
  Guest path: /var/lib/docker/docker_1437748320_29948
  Command: rsync --verbose --archive --delete -z --copy-links --no-owner --no-group --rsync-path sudo rsync -e ssh -p 2222 -o StrictHostKeyChecking=no -o IdentitiesOnly=true -o UserKnownHostsFile=/dev/null --exclude .vagrant/ /Users/d022051/tmp/vagranttest/ docker@127.0.0.1:/var/lib/docker/docker_1437748320_29948
  Error: Warning: Permanently added '[127.0.0.1]:2222' (RSA) to the list of known hosts.
  Connection closed by 127.0.0.1
  rsync: connection unexpectedly closed (0 bytes received so far) [sender]
  rsync error: unexplained error (code 255) at io.c(226) [sender=3.1.1]
  </code></pre>
  
  <p>And this is the relevant part from the Vagrant configuration:</p>
  
  <pre><code>  config.vm.provider "docker" do |d|
      d.image = "bowline/orientdb:dev"
    end
  
    config.vm.synced_folder ".", "/vagrant", type: "rsync"
  </code></pre>
  
  <p>Is there a default password for the docker user? Do I have to create the docker user in my docker image? </p>
  
  <p><strong>UPDATE:</strong> After a reboot of my Mac the problem is transformed. Now the problem is that vagrant try to install rsync on the VM and fails. So consider this question closed, as I currently cannot reproduce the original issue.</p>