<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _get_dcd(self, alias):
 """ Get the Docker-Content-Digest header for an alias. :param alias: Alias name. :type alias: str :rtype: str :returns: DCD header for the alias. """

        # https://docs.docker.com/registry/spec/api/#deleting-an-image
        # Note When deleting a manifest from a registry version 2.3 or later,
        # the following header must be used when HEAD or GET-ing the manifest
        # to obtain the correct digest to delete:
        # Accept: application/vnd.docker.distribution.manifest.v2+json
        return self._request(
            'head',
            'manifests/{}'.format(alias),
            headers={'Accept': _schema2_mimetype},
        ).headers.get('Docker-Content-Digest')