--provider=docker时出现的问题

Vagrant.configure("2") do |config|
  config.vm.box = "hashicorp/precise64"
  config.vm.network "forwarded_port", guest: 6379, host: 6379
  config.vm.provision "docker"
  config.vm.provision "shell", inline: <<-SHELL
    "ps aux | grep ‘sshd:‘ | awk ‘{print $2}‘ | xargs kill"
  SHELL
end

userdeMBP:~ user$ vagrant up --provider=docker
An active machine was found with a different provider. Vagrant
currently allows each machine to be brought up with only a single
provider at a time. A future version will remove this limitation.
Until then, please destroy the existing machine to up with a new
provider.

Machine name: default
Active provider: virtualbox
Requested provider: docker