New walker for walker_nav_menu to change inside container data

Well, you wouldn’t need a walker for that, if it’s simply adding a data attribute or something like that you could just use the items_wrap parameter. View wp_nav_menu() in Resources:

$items_wrap = '<div class="top-nav" mine="something">';
$items_wrap .= '<ul id="%1$s" mine="something2" class="nav %2$s">%3$s</ul>';
$items_wrap .= '</div>';

wp_nav_menu( array(
    'container'         => false,
    'echo'              => true,
    'before'            => '',
    'after'             => '',
    'link_before'       => '',
    'link_after'        => '',
    'depth'             => 0,
    'theme_location'    => 'topnav',
    'items_wrap'        => $items_wrap,     // The Default is: '<ul id="%1$s" class="%2$s">%3$s</ul>'
    'walker'            => new description_walker()
) );

This is a more straight-froward way to do it than messing with a walker or something of the sort.