Save image as featured image

I am using WordPress 4.9.7 and I am trying to develop a plugin that downloads an image from a website and saves it in the media library and sets it as featured image on a post.

Find below my full plugin code for saving the image:

<?php

/*
Plugin Name: Custom Importer Coin Data
Author: Test
*/

add_action('admin_menu', 'button_menu');

function button_menu()
{
    add_menu_page('Import Coin Page', 'Import Coins from CMC/WTM', 'manage_options', 'coinmarketcapimporter-slug', 'importCoinMarketCap_admin_page');

}

function importCoinMarketCap_admin_page()
{

    if (!current_user_can('manage_options')) {
        wp_die(__('You do not have sufficient privilege to access this page.'));
    }

    echo '<div class="wrap">';
    echo '<h2>Import Data</h2>';

    // Check whether the button has been pressed AND also check the nonce
    if (isset($_POST['importCoinMarketCap']) && check_admin_referer('importCoinMarketCap_clicked')) {
        // the button has been pressed AND we've passed the security check
        importAction();
    }

    echo '<form action="options-general.php?page=coinmarketcapimporter-slug" method="post">';

    wp_nonce_field('importCoinMarketCap_clicked');
    echo '<input type="hidden" value="true" name="importCoinMarketCap" />';
    submit_button('Import CoinMarketCap/WhatToMine Coins');
    echo '</form>';

    echo '</div>';

}

function importAction()
{
    $COIN_MARKET_CAP_URL = 'https://s2.coinmarketcap.com/generated/search/quick_search.json';
    $WHAT_TO_MINE_URL = 'https://whattomine.com/calculators.json';

    echo '<div id="message" class="updated fade"><p>' . 'The "Call Function" button was clicked.' . '</p></div>';

    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, $COIN_MARKET_CAP_URL);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    $output = curl_exec($ch);
    curl_close($ch);
    $output = str_replace("'", "", $output);
    $outputdecoded = json_decode($output, true);

    $c = curl_init();
    curl_setopt($c, CURLOPT_URL, $WHAT_TO_MINE_URL);
    curl_setopt($c, CURLOPT_RETURNTRANSFER, 1);
    $outputWTM = curl_exec($c);
    curl_close($c);
    $outputWTM = str_replace("'", "", $outputWTM);
    $outputdecodedWTM = json_decode($outputWTM, true);

    echo '<b>Start import</b>';
    echo '<ul>';
    foreach ($outputdecodedWTM['coins'] as $key => $wtm) {
        $algo = $wtm['algorithm'];

        foreach ($outputdecoded as $coin) {
            if($wtm['tag'] == $coin['symbol']){

                // Check if the post already exists
                if (!post_exists($coin['name'])) {
                    // Get CMC ID
                    $cmcid = $coin["id"];
                    // Get public ID
                    $pubid = $coin["slug"];
                    // Using the CMC ID build the link for logo
                    $file_url = "https://s2.coinmarketcap.com/static/img/coins/128x128/" . $cmcid . ".png";

                    // insert the post
                    echo ('<li>' . $pubid . '</li>');

                    $postId = wp_insert_post(array(
                        'post_type' => 'coin',
                        'post_status' => 'publish',
                        'post_title' => sanitize_textarea_field($coin['name']),
                        'post_name' => sanitize_textarea_field($coin['slug']),
                        'meta_input' => array(
                            'symbol' => sanitize_textarea_field($wtm['tag']),
                            'coin_algorithm' => sanitize_textarea_field($algo),
                        ),
                    ));

                    // get image
                    $uploaddir  = wp_upload_dir();
                    $filename = $uploaddir['path'] . "https://wordpress.stackexchange.com/" . $coin["slug"]  . '.png';

                    $contents = file_get_contents($file_url);
                    $savefile = fopen($filename, 'w');
                    fwrite($savefile, $contents);
                    fclose($savefile);

                    // add image to media library
                    $wp_filetype = wp_check_filetype(basename($filename), null);

                    $attachment = array(
                        'post_mime_type' => $wp_filetype['type'],
                        'post_title' => $filename,
                        'post_content' => '',
                        'post_status' => 'inherit'
                    );

                    $thumbnail_id = wp_insert_attachment($attachment, $filename);
                    set_post_thumbnail($postId, $thumbnail_id);
                }
            }
        }
    }
    echo '</ul>';
    echo '<b>End import</b>';
}
?>

The download and upload to the image path works perfectly on my dev server, however, when I upload the plugin to the production server, I cannot load the image correctly.

On the production server, the image cannot be found as the file path seems to be wrong:

Am I saving the image wrongly or with the wrong file path?