fix: docker add SECURE_PROXY_SSL_HEADER env config

[sorvis]

Found that when running in Kubernetes behind an nginx proxy that adds https then was seeing the following error in the logs:

[Fri May 17 13:51:18.518462 2024] [wsgi:error] [pid 28:tid 136755443365632] [remote 10.20.2.3:39312] ERROR CSRF failure detected
[Fri May 17 13:51:18.518699 2024] [wsgi:error] [pid 28:tid 136755443365632] [remote 10.20.2.3:39312] INFO Forbidden (Origin checking failed - https://frepple.kencove.com/ does not match any trusted origins.): /data/login/

See also: https://stackoverflow.com/questions/70501974/django-returning-csrf-verification-failed-request-aborted-behind-nginx-prox

As a fix added a variable _SECURE_PROXY_SSLHEADER when set it will un-comment that line in the config file.

[jdetaeye]

I prefer not to merge this one:

• Topic is clearly explained in the djangosettings.py file and in the documentation https://frepple.com/docs/current/installation-guide/advanced-configuration.php#proxy This is not something new.
• It's simply impossible and not wise to cover all possible deployment options with environment variables. Instead the recommended approach is to create your own dockerfile that inherits from the standard frepple image and adds any configuration&customization you need.