Fix nginx set_real_ip_from conf

[rimi-itk]

https://leantime.itkdev.dk/_#/tickets/showTicket/6275

• Replaces "${NGINX_SET_REAL_IP_FROM:-192.168.39.0/24}" with 192.168.39.0/24 in nginx conf templates. See below for details on why.
• Adds simple nginx conf validation

The details

Using ${name:-…} to set a default value for a non-existing (or empty) environment variable does not work in nginx templates:

$ (cd templates/symfony-6; COMPOSE_DOMAIN=example.com docker compose run --no-deps --rm nginx nginx -t)
…
2026/01/07 11:58:37 [emerg] 1#1: host not found in set_real_ip_from "${NGINX_SET_REAL_IP_FROM:-192.168.39.0/24}" in /etc/nginx/conf.d/default.conf:10
nginx: [emerg] host not found in set_real_ip_from "${NGINX_SET_REAL_IP_FROM:-192.168.39.0/24}" in /etc/nginx/conf.d/default.conf:10
nginx: configuration file /etc/nginx/nginx.conf test failed

Notice "${NGINX_SET_REAL_IP_FROM:-192.168.39.0/24}" is reported, i.e. ${NGINX_SET_REAL_IP_FROM:-192.168.39.0/24} is not replaced with 192.168.39.0/24 as hoped (and expected due to it working in many other places).

It's the nginx docker image that substitutes variables in templates using envsubst, cf. https://github.com/docker-library/docs/tree/master/nginx#using-environment-variables-in-nginx-configuration-new-in-119, but envsubst does not support default values for non-existing variables:

$ (cd templates/symfony-6; COMPOSE_DOMAIN=example.com docker compose run --no-deps --rm nginx sh -c 'envsubst < /etc/nginx/templates/default.conf.template')
server {
    listen 8080;
    server_name localhost;

    root /app/public;

    client_max_body_size 5M;

    set_real_ip_from 172.16.0.0/16;
    set_real_ip_from ${NGINX_SET_REAL_IP_FROM:-192.168.39.0/24};
…

[turegjorup]

Approved with comment

[turegjorup]

Please use a designated example domain:

Reserved Example Second Level Domain Names

The Internet Assigned Numbers Authority (IANA) also currently has the following second level domain names reserved which can be used as examples.

    example.com
    example.net
    example.org

RFC 2606 Reserved Top Level DNS Names June 1999

[rimi-itk]

Fixed in dcc85d7.