Fix nginx WebSocket and X-Forwarded-Proto handling for all deployment scenarios

[GhostInTheNN]

Proposed Changes

• Added map directive to nginx.conf for universal X-Forwarded-Proto handling (works with/without upstream reverse proxy)
• Added WebSocket support headers (Upgrade, Connection) to nginx.conf
• Extended timeouts for WebSocket connections in nginx.conf
• Removed --proxy-protocol True from GUNICORN_CMD_ARGS in prod.env (incompatible with standard HTTP from nginx)
• Added pytest test suite (7 tests with value validation) to validate nginx configuration
• Added mock backend server for header validation testing
• Added Docker Compose test environment
• Added GitHub Actions CI/CD workflow for automated testing
• Added test documentation in tests/README.md

Related Issue(s)

Closes wger-project/docker#138

Background

I encountered issues integrating wger into my home lab environment using the default docker configuration. After investigation, I identified two issues that affect deployments across various network configurations:

1. Missing X-Forwarded-Proto fallback - The nginx configuration didn't handle scenarios without an upstream reverse proxy, causing Django CSRF validation failures
2. Gunicorn PROXY protocol mismatch - The --proxy-protocol True flag was incompatible with nginx's standard HTTP communication

Testing

All changes have been validated locally and pass the test suite:

cd tests
docker compose -f docker-compose.test.yml up --abort-on-container-exit test
# Result: 7 passed

Tests cover:

• Reverse proxy scenarios (HTTP/HTTPS preservation)
• Direct connection fallback
• WebSocket header proxying
• Standard proxy headers (X-Real-IP, X-Forwarded-For, Host)

Deployment Scenarios Supported

✅ Behind reverse proxy (Traefik, Caddy, nginx) ✅ Direct connection ✅ Port forwarding with/without reverse proxy ✅ WebSocket connections

These changes are backwards compatible and don't break existing deployments.

[rolandgeider]

@GhostInTheNN I just now opened the pr, I had thought it was just a missing header or something like that, but you included a whole test suite! I'll properly look at it next ween when I'll have time