Example multisite local settings file.

default.local.settings.php

<?php

/**
 * @file
 * Local development override configuration feature.
 */

use Symfony\Component\Yaml\Yaml;

if (file_exists($dir . '/blt/project.yml')) {
  $project = Yaml::parse(file_get_contents($dir . '/blt/project.yml'));
  //$request = \Drupal::hasRequest() ? \Drupal::request() : \Symfony\Component\HttpFoundation\Request::createFromGlobals();
  //$role = basename(\Drupal\Core\DrupalKernel::findSitePath($request));
  $name = substr($_SERVER['HTTP_HOST'],0, strpos($_SERVER['HTTP_HOST'],'.local'));

  if (isset($project['project']['acsf'])
    && !empty($project['multisites'])
    && array_key_exists($name, $project['multisites'])) {
    // The DB role will be the same as the gardens site directory name.
    /**
     * Database configuration.
     */
    $databases = array(
      'default' =>
        array(
          'default' =>
            array(
              'database' => "drupal_{$name}",
              'username' => 'drupal',
              'password' => 'drupal',
              'host' => 'localhost',
              'port' => '3306',
              'namespace' => 'Drupal\\Core\\Database\\Driver\\mysql',
              'driver' => 'mysql',
              'prefix' => '',
            ),
        ),
    );

    $config_directories['sync'] = $dir . '/config/' . $name . '/local';

    /**
     * Private file path.
     */
    $settings['file_private_path'] = $dir . "/files-private/${name}";

    /**
     * Public file path.
     */
    // Set public filesystem per local development site.
    $settings['file_public_path'] = "sites/default/files/${name}";
  }
}
else {
  $databases = array(
    'default' =>
      array(
        'default' =>
          array(
            'database' => 'drupal',
            'username' => 'drupal',
            'password' => 'drupal',
            'host' => 'localhost',
            'port' => '3306',
            'namespace' => 'Drupal\\Core\\Database\\Driver\\mysql',
            'driver' => 'mysql',
            'prefix' => '',
          ),
      ),
  );

  $config_directories['sync'] = $dir . '/config/default';

  // Set private filesystem for local development.
  $settings['file_private_path'] = $dir . "/files-private/default";
}

// Use development service parameters.
$settings['container_yamls'][] = $dir . '/docroot/sites/development.services.yml';

// Allow access to update.php.
$settings['update_free_access'] = TRUE;

/**
 * Assertions.
 *
 * The Drupal project primarily uses runtime assertions to enforce the
 * expectations of the API by failing when incorrect calls are made by code
 * under development.
 *
 * @see http://php.net/assert
 * @see https://www.drupal.org/node/2492225
 *
 * If you are using PHP 7.0 it is strongly recommended that you set
 * zend.assertions=1 in the PHP.ini file (It cannot be changed from .htaccess
 * or runtime) on development machines and to 0 in production.
 *
 * @see https://wiki.php.net/rfc/expectations
 */
assert_options(ASSERT_ACTIVE, TRUE);
\Drupal\Component\Assertion\Handle::register();

/**
 * Show all error messages, with backtrace information.
 *
 * In case the error level could not be fetched from the database, as for
 * example the database connection failed, we rely only on this value.
 */
$config['system.logging']['error_level'] = 'verbose';

/**
 * Disable CSS and JS aggregation.
 */
$config['system.performance']['css']['preprocess'] = FALSE;
$config['system.performance']['js']['preprocess'] = FALSE;

/**
 * Disable the render cache (this includes the page cache).
 *
 * Note: you should test with the render cache enabled, to ensure the correct
 * cacheability metadata is present. However, in the early stages of
 * development, you may want to disable it.
 *
 * This setting disables the render cache by using the Null cache back-end
 * defined by the development.services.yml file above.
 *
 * Do not use this setting until after the site is installed.
 */
# $settings['cache']['bins']['render'] = 'cache.backend.null';

/**
 * Disable Dynamic Page Cache.
 *
 * Note: you should test with Dynamic Page Cache enabled, to ensure the correct
 * cacheability metadata is present (and hence the expected behavior). However,
 * in the early stages of development, you may want to disable it.
 */
# $settings['cache']['bins']['dynamic_page_cache'] = 'cache.backend.null';

/**
 * Allow test modules and themes to be installed.
 *
 * Drupal ignores test modules and themes by default for performance reasons.
 * During development it can be useful to install test extensions for debugging
 * purposes.
 */
$settings['extension_discovery_scan_tests'] = FALSE;

/**
 * Enable access to rebuild.php.
 *
 * This setting can be enabled to allow Drupal's php and database cached
 * storage to be cleared via the rebuild.php page. Access to this page can also
 * be gained by generating a query string from rebuild_token_calculator.sh and
 * using these parameters in a request to rebuild.php.
 */
$settings['rebuild_access'] = FALSE;

/**
 * Temporary file path:
 *
 * A local file system path where temporary files will be stored. This
 * directory should not be accessible over the web.
 *
 * Note: Caches need to be cleared when this value is changed.
 *
 * See https://www.drupal.org/node/1928898 for more information
 * about global configuration override.
 */
$config['system.file']['path']['temporary'] = '/tmp';

/**
 * Trusted host configuration.
 *
 * See full description in default.settings.php.
 */
$settings['trusted_host_patterns'] = array(
  '^.+$',
);