See extension: https://github.com/extcode/mail_conf_override/

Override TYPO3 ext:mail_conf_override settings for mailhog in an specific applicationContext.php

<?php
if (!defined('TYPO3_MODE')) {
	die ('Access denied.');
}

// Override configuration of LocalConfiguration
$customChanges = [
	'GFX' => [
		'TTFdpi' => '96',
		'colorspace' => 'RGB',
		'gdlib_png' => '1',
		'im' => 1,
		'im_mask_temp_ext_gif' => 1,
		'im_path' => '/usr/bin/',
		'im_path_lzw' => '/usr/bin/',
		'im_v5effects' => -1,
		'im_version_5' => 'gm',
		'image_processing' => 1,
		'jpg_quality' => '80',
		'png_truecolor' => '1',
    ],
    // Configuration for local mailhog container (running as "mail" docker container)
    'MAIL' => [
        'transport' => 'smtp',
        'transport_smtp_encrypt' => '',
        'transport_smtp_password' => '',
        'transport_smtp_server' => 'mail:1025',
        'transport_smtp_username' => '',
    ],
];
// Override ext:mail_conf_override settings for local mailhog container
foreach ($GLOBALS['TYPO3_CONF_VARS']['MAIL']['overrides'] as $fromAddress => $settings) {
    $customChanges['MAIL']['overrides'][$fromAddress]['transport_smtp_encrypt'] = '';
    $customChanges['MAIL']['overrides'][$fromAddress]['transport_smtp_password'] = '';
    $customChanges['MAIL']['overrides'][$fromAddress]['transport_smtp_server'] = 'mail:1025';
    $customChanges['MAIL']['overrides'][$fromAddress]['transport_smtp_username'] = '';
}

$GLOBALS['TYPO3_CONF_VARS'] = array_replace_recursive($GLOBALS['TYPO3_CONF_VARS'], $customChanges);