CodeIgniter 3 + Docker - 会话

I'm migrating my CodeIgniter 3 app into Docker and I have session issue. I can't login to app, because of CI sessions.

It seems like CodeIgniter recreates new session with each request to the server. When I checked what $_COOKIE contains i can see two fields:

PHPSESSID - always the same (i even tried manually start php session)
ci_session - changes with every request

Files for new sessions are created by www-data user and have right permissions.

Session config:

$config['base_url'] = 'http://localhost';

$config['sess_driver'] = 'files';
$config['sess_cookie_name'] = 'ci_session';
$config['sess_expiration'] = 7200;
$config['sess_save_path'] = sys_get_temp_dir();
$config['sess_match_ip'] = FALSE;
$config['sess_time_to_update'] = 300;
$config['sess_regenerate_destroy'] = FALSE;

$config['cookie_prefix']    = '';
$config['cookie_domain']    = '';
$config['cookie_path']      = '/';
$config['cookie_secure']    = FALSE;
$config['cookie_httponly']  = FALSE;

Docker images: php:7.1-fpm, nginx:latest, mysql:latest

I haven't idea what's going on. Can anybody help me?

OK, problem solved.

I must update CodeIgniter to older version, because of currently installed isn't compatible with PHP 7.1 (only with 7.0).