def _get_dcd(self, alias):
<EOL>return self._request(<EOL>'<STR_LIT>',<EOL>'<STR_LIT>'.format(alias),<EOL>headers={'<STR_LIT>': _schema2_mimetype},<EOL>).headers.get('<STR_LIT>')<EOL>
Get the Docker-Content-Digest header for an alias.

:param alias: Alias name.
:type alias: str

:rtype: str
:returns: DCD header for the alias.