Implement template for SSSD configuration

[runleveldev]

Copilot Summary

This pull request updates the SSSD configuration process in the base image to support environment-based templating, making the configuration more flexible and secure. The static sssd.conf is replaced with a template that is rendered at container startup using environment variables, and the Dockerfile and systemd configuration are updated accordingly.

SSSD configuration templating:

• Replaced the static sssd.conf file with a new sssd.conf.template that uses environment variables for all sensitive or environment-specific settings, such as LDAP URIs, TLS requirements, schema, search bases, and authentication tokens. [1] [2]
• Updated the Dockerfile to copy the template (sssd.conf.template) instead of the static config, and added a systemd drop-in (50-sssd-conf-template.conf) to render the template into the final sssd.conf at service startup using envsubst. [1] [2]