title: How do I get the AccountId as a variable in a serverless.yml file?
body: <p>I want to build an ARN in my file dynamically, but I need to get my current AccountId. How can I access it as a variable?</p>
  <p>For example:</p>
  <pre><code>example: arn:aws:states:${region}:${accountId}:stateMachine:${self:service}-${self:custom.stage}-example
  </code></pre>
  <p>What is the proper way to reference the current <code>region</code> and <code>accountId</code>?</p>