composer.json

{
  "autoload": {
    "classmap": [
      "scripts/composer/PrivateAuth.php"
    ]
  },
  "scripts": {
    "loadPrivateAuth": "Project\\composer\\ComposerPrivateAuth::loadComposerSecrets",
    "pre-install-cmd": "@loadPrivateAuth",
    "pre-update-cmd": "@loadPrivateAuth"
  }
}

ComposerPrivateAuth.php

<?php
/**
 * @file
 * Contains \Project\composer\ComposerPrivateAuth.
 */

namespace Project\composer;

use Composer\Script\Event;

class ComposerPrivateAuth
{

    /**
     * Load environmental variables from private path.
     */
    public static function loadComposerSecrets(Event $event):
        void
        {
            // Load config into composer
            $PACKAGIST_HOST = getenv('PACKAGIST_HOST');
            $PACKAGIST_USER = getenv('PACKAGIST_USER');
            $PACKAGIST_PASSWORD = getenv('PACKAGIST_PASSWORD');

            $config = ["type" => "composer", "url" => "https://$PACKAGIST_USER:$PACKAGIST_PASSWORD@$PACKAGIST_HOST"];

            // Add auth to repositories.
            $repositoryManager = $event->getComposer()
                ->getRepositoryManager();
            $repository = $repositoryManager->createRepository('composer', $config, $PACKAGIST_HOST);
            $repositoryManager->prependRepository($repository);
        }
    }