Add custom Nginx entrypoint to wait for Wazuh upstreams before start

[azeddine-hmd]

This PR introduces a custom Nginx startup script that ensures Wazuh upstream nodes are reachable before Nginx begins serving traffic.
The change improves reliability during multi-node deployments, avoiding potential startup race conditions between Nginx and Wazuh services.

Changes

Added nginx-entrypoint-check.sh script:
Waits for connectivity to:
wazuh-master:1514
wazuh-worker:1514
Starts Nginx only after both are reachable.
Updated multi-node/docker-compose.yml:
Mounted the script into the Nginx container.
Added runtime installation of netcat-openbsd (required for network checks).
Adjusted container command to execute the custom entrypoint.

Future Improvements

I can move the netcat-openbsd installation to a build-time step in a custom Nginx image for faster and more deterministic startup. Additionally, replacing apt update at runtime with a prebuilt image for production use.