Returns a list of tagged images

    CLI Example:

    .. code-block:: bash

        salt myminion docker.list_tags

def list_tags():
    '''
    Returns a list of tagged images

    CLI Example:

    .. code-block:: bash

        salt myminion docker.list_tags
    '''
    ret = set()
    for item in six.itervalues(images()):
        if not item.get('RepoTags'):
            continue
        ret.update(set(item['RepoTags']))
    return sorted(ret)