Custom Post Type & Page Template

You should NOT use flush_rewrite_rules when you register your custom post type.
Doing so means you will flush permalinks on every page load.
Instead you should use the register_activation_hook.

Re the template issue: Try renaming the template to single-dossier.php

And try changing your register_post_type array to this:

register_post_type( 'dossier' , array(
    'label' => __('Dossiers'),
    'singular_label' => __('Dossier'),
    'public' => true,
    'show_ui' => true,
    'query_var' => true,
    'rewrite' => array( 'slug' => 'dossier' ),
    'capability_type' => array('dossier', 'dossiers'),
    'hierarchical' => true,
    'has_archive' => false,
    'menu_position' => 3,
    'supports' => array('title', 'editor', 'thumbnail', 'page-attributes')
   ));