title: Is there a `ssh-add` Linux alpine one liner
body: <p>I need during a Gitlab-CI build to authenticate with ssh-agent from an alpine image.</p>
  
  <p>I am looking for a <strong>sh</strong> one liner equivalent of this <strong>bash</strong> command <a href="https://docs.gitlab.com/ee/ci/ssh_keys/README.html#ssh-keys-when-using-the-docker-executor" rel="noreferrer">(picked from the gitlab documentation)</a>: </p>
  
  <pre><code>ssh-add &lt;(echo "$SSH_PRIVATE_KEY")
  </code></pre>
  
  <p>I have tried : </p>
  
  <pre><code>echo $SSH_PRIVATE_KEY | ssh-add -
  Enter passphrase for (stdin): ERROR: Job failed: exit code 1
  
  printf '%s\n' "$SSH_PRIVATE_KEY" | ssh-add
  ERROR: Job failed: exit code 1
  </code></pre>