I want to add class to caption shortcode, pulled from custom attachment field. I add the field via:

    function add_img_class( $form_fields, $post ) {
        $form_fields['img_class_field'] = array(
            'label' => 'External Link Title',
            'input' => 'text',
            'value' => get_post_meta( $post->ID, 'img_class_field', true )
        );
        return $form_fields;
    }

    add_filter( 'attachment_fields_to_edit', 'add_img_class', 10, 2 );
    
    
    function save_img_class( $post, $attachment ) {
        if( isset( $attachment['img_class_field'] ) )
            update_post_meta( $post['ID'], 'img_class_field', $attachment['img_class_field']);
        return $post;
    }

    add_filter( 'attachment_fields_to_save', 'save_img_class', 10, 2 );

How do I dynamically add "class" attribute with the field value to caption shortcode now? 

    [caption ... class="img_class_field"]<img src...>[/caption]