How do I make Cloudfiles FormPost return the "Access-Control-Allow-Origin" header to enable CORS?

Question

Rackspace has docs on how to enable CORS for a container here. Example 7.11 is a CORS Test Page that'll let you test your configuration outside of attempting file uploads.

Test CORS Page - accepts a token and a URL to an object or container and let's you try calling an HTTP method on it.

NOTE: I've removed the "DELETE" method as that can have undesired results (Such as deleting your container/object)

<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <title>Test CORS</title>
  </head>
  <body>

    Token<br><input id="token" type="text" size="64"><br><br>

    Method<br>
    <select id="method">
        <option value="GET">GET</option>
        <option value="HEAD">HEAD</option>
        <option value="POST">POST</option>
        <option value="PUT">PUT</option>
    </select><br><br>

    URL (Container or Object)<br><input id="url" size="64" type="text"><br><br>

    <input id="submit" type="button" value="Submit" onclick="submit(); return false;">

    <pre id="response_headers"></pre>
    <p>
    <hr>
    <pre id="response_body"></pre>

    <script type="text/javascript">
      function submit() {
          var token = document.getElementById('token').value;
          var method = document.getElementById('method').value;
          var url = document.getElementById('url').value;

          document.getElementById('response_headers').textContent = null;
          document.getElementById('response_body').textContent = null;

          var request = new XMLHttpRequest();

          request.onreadystatechange = function (oEvent) {
              if (request.readyState == 4) {
                  responseHeaders = 'Status: ' + request.status;
                  responseHeaders = responseHeaders + '\nStatus Text: ' + request.statusText;
                  responseHeaders = responseHeaders + '\n\n' + request.getAllResponseHeaders();
                  document.getElementById('response_headers').textContent = responseHeaders;
                  document.getElementById('response_body').textContent = request.responseText;
              }
          }

          request.open(method, url);
          request.setRequestHeader('X-Auth-Token', token);
          request.send(null);
      }
    </script>

  </body>
</html>

To set these values outside of pyrax or any other SDK, I've used the following code:

https://gist.github.com/chrisrasco/7455804

Remember to set your username, apikey, and path to your container in the appropriate places.