How do I add my own public key to Vagrant VM?

You can use Ruby's core File module, like so:

  config.vm.provision "shell" do |s|
    ssh_pub_key = File.readlines("#{Dir.home}/.ssh/id_rsa.pub").first.strip
    s.inline = <<-SHELL
      echo #{ssh_pub_key} >> /home/vagrant/.ssh/authorized_keys
      echo #{ssh_pub_key} >> /root/.ssh/authorized_keys
    SHELL
  end

This working example appends ~/.ssh/id_rsa.pub to the ~/.ssh/authorized_keys of both the vagrant and root user, which will allow you to use your existing SSH key.

Copying the desired public key would fall squarely into the provisioning phase. The exact answer depends on what provisioning you fancy to use (shell, Chef, Puppet etc). The most trivial would be a file provisioner for the key, something along this:

config.vm.provision "file", source: "~/.ssh/id_rsa.pub", destination: "~/.ssh/me.pub"

Well, actually you need to append to authorized_keys. Use the the shell provisioner, like so:

Vagrant.configure(2) do |config|
  # ... other config
  config.vm.provision "shell", inline: <<-SHELL
    cat /home/vagrant/.ssh/me.pub >> /home/vagrant/.ssh/authorized_keys
  SHELL
  # ... other config
end

You can also use a true provisioner, like Puppet. For example see Managing SSH Authorized Keys with Puppet.

There's a more "elegant" way of accomplishing what you want to do. You can find the existing private key and use it instead of going through the trouble of adding your public key.

Proceed like this to see the path to existing private key (look below for IdentityFile):

run

 vagrant ssh-config 

result:

$ vagrant ssh-config
Host magento2.vagrant150
  HostName 127.0.0.1
  User vagrant
  Port 3150
  UserKnownHostsFile /dev/null
  StrictHostKeyChecking no
  PasswordAuthentication no
  IdentityFile "/Users/madismanni/m2/vagrant-magento/.vagrant/machines/magento2.vagrant150/virtualbox/private_key"
  IdentitiesOnly yes
  LogLevel FATAL

Then you can use the private key like this, note also the switch for switching off password authentication

ssh -i /Users/madismanni/m2/vagrant-magento/.vagrant/machines/magento2.vagrant150/virtualbox/private_key -o PasswordAuthentication=no [email protected] -p 3150