title: Unable to ssh to app engine instance from docker
body: <p>Trying to connect via SSH to an instance from inside a docker image.</p>
  <pre><code>&gt;&gt;&gt; gcloud app instances ssh aef-default-vb6e0b04-054k --quiet --service default --version vb6e0b04 --project myproject --container=&quot;gaeapp&quot; -- echo &quot;OK&quot;
  
  Waiting for operation [apps/myproject/operations/856b8bb4-c359-4cee-937c-1b92f7218b11] to complete...done.
  DEBUG: Running command [/usr/bin/ssh -T -i /root/.ssh/google_compute_engine -o CheckHostIP=no -o HostKeyAlias=gae.myproject.aef-default-vcb4f1c9-frn7 -o IdentitiesOnly=yes -o UserKnownHostsFile=/root/.ssh/google_compute_known_hosts root@35.187.107.247 -- sudo docker exec -i gaeapp echo OK].
  DEBUG: Executing command: ['/usr/bin/ssh', '-T', '-i', '/root/.ssh/google_compute_engine', '-o', 'CheckHostIP=no', '-o', 'HostKeyAlias=gae.myproject.aef-default-vcb4f1c9-frn7', '-o', 'IdentitiesOnly=yes', '-o', 'UserKnownHostsFile=/root/.ssh/google_compute_known_hosts', 'root@35.187.107.247', '--', 'sudo', 'docker', 'exec', '-i', 'gaeapp', 'echo', 'OK']
  Warning: Permanently added 'gae.myproject.aef-default-vcb4f1c9-frn7' (ED25519) to the list of known hosts.
  root@35.187.107.247: Permission denied (publickey).
  DEBUG: (gcloud.app.instances.ssh) [/usr/bin/ssh] exited with return code [255].
  </code></pre>