title: How do I add a SSH key to Google Cloud Compute Engine VM Linux instance?
body: <p>I have a Linux VM instance running in Google Cloud Platform.
  I tried to copy my public key to <code>~/.ssh/authorized_keys</code> and I can successfully SSH to my VM.</p>
  
  <p>But sometimes <code>~/.ssh/authorized_keys</code> is flushed and I have to copy the public key again.
  It is really a pain to add public key every time.
  How do I add a public key permanently?</p>