title: Error loading key "(stdin)": invalid format Gitlab CI
body: <p>This is piece of my yaml file about ssh for gitlab ci:</p>
  
  <pre><code>eval $(ssh-agent -s)
  echo "$SSH_PRIVATE_KEY" | tr -d '\r' | ssh-add - &gt; /dev/null
  mkdir -p ~/.ssh
  chmod 700 ~/.ssh
  '[[ -f /.dockerenv ]] &amp;&amp; echo -e "Host *\n\tStrictHostKeyChecking no\n\n" &gt; ~/.ssh/config'
  </code></pre>
  
  <p>When I attempted to set SSH_PRIVATE_KEY variable to protected state, it occurs error:</p>
  
  <blockquote>
    <p>Error loading key "(stdin)": invalid format</p>
  </blockquote>
  
  <p>and I don't know why it's okay when I set SSH_PRIVATE_KEY variable to unprotected state</p>