Steps to reproduce:

1. Install Zabbix suite as docker services (zabbix-web-nginx-mysql flavour, based on alpine OS)

Result:
The web server complains about not being able to connect to the zabbix server:

Solution:
Zabbix frontend retrieves the IP address of zabbix server (ref. zabbix.conf.php) via the ZBX_SERVER_HOST environment variable.
But that variable is not set initially set, and defaults to 'localhost' value.
As a sulution, env_vars/.env_web file should have

ZBX_SERVER_HOST=zabbix-server

set, by default.