Hello,
When setting up the samlsso plug-in, it has a Wiki page that states issues with Docker and Reverse Proxy environments, as the GLPI Docker image comes with only HTTP port enabled and enabling the PHP directive session.cookie_secure breaks GLPI with the error message:
The web server is configured to allow session cookies only in a secure context (HTTPS). Therefore, you must access GLPI in a secure context to be able to use it.
It happens because PHP Symphony doesn't trust the reverse proxy. A workaround for this message on Docker is setting up this trust manually, as seen in DonutsNL/samlsso#59.
Another solution would be that the GLPI Docker container also comes with a self-signed certificate and listens on HTTPS port.