Update OIDC Docs for IdP alias guidance for self-hosted OIDC behind Pangolin - Issue #3

[marcschaeferger]

Summary by Copilot:

This pull request adds documentation to clarify how to configure internal DNS resolution for a self-hosted OpenID Connect (OIDC) provider when it is hosted behind the same Pangolin/Traefik stack. The new guidance helps ensure that service-to-service OIDC flows work correctly within the Docker Compose network by using network aliases for the IdP domain. This avoids issues where containers cannot resolve the IdP hostname internally, which can lead to authentication failures.

OIDC Internal DNS Configuration:

• Added instructions for adding the IdP domain as a Docker network alias on the service exposing ports 80/443 (typically gerbil or traefik) to ensure internal callbacks and token/userinfo requests resolve correctly within the stack. [1] [2]
• Provided scenario-based examples for both cases: when Traefik shares the gerbil network namespace and when Traefik runs in its own namespace, including sample docker-compose.yaml snippets. [1] [2]
• Clarified that public DNS remains unchanged and the alias only affects internal container DNS resolution for OIDC flows. [1] [2]
• Added troubleshooting tips for DNS errors and guidance on when to remove the alias, emphasizing best practices to avoid edge cases with OIDC authentication. [1] [2]

Closes #3