set Craft to use local mailhog instance. Just drop in an existing plugin's init function, refresh and then remove (assumes you're using official Mailhog docker instance

index.php

<?php
  $mailhog_name = getenv('MAILHOG_NAME');
  $mailhog_port = getenv('MAILHOG_PORT');
  if ($mailhog_name && $mailhog_port) {
      $hostname = array_slice(explode('/', getenv('MAILHOG_NAME')), -1, 1)[0];
      $port = array_slice(explode(':', getenv('MAILHOG_PORT')), -1, 1)[0];
      $settings = craft()->systemSettings->getSettings('email');
      $settings['protocol'] = 'smtp';
      $settings['port'] = $port;
      $settings['host'] = $hostname;
      $settings['timeout'] = 10;
      $settings['smtpKeepAlive'] = '';
      $settings['smtpAuth'] = '';
      $settings['smtpSecureTransportType'] = 'none';
      craft()->systemSettings->saveSettings('email', $settings);
      echo "Settings updated to use mailhog\n";
  } else {
      echo "Mailhog environment variables not found, skipping\n";
  }