Fix NPM forwarding: use HOST_IP env var instead of socket detection

Socket detection inside Docker returns the container IP (172.18.0.x),
not the host IP. Now:
- install.sh detects host IP via hostname -I and stores in .env
- docker-compose.yml passes HOST_IP to the container
- npm_service.py reads HOST_IP from environment
- Increased SSL cert timeout to 120s (LE validation is slow)
- Added better logging for SSL cert creation/assignment
- README updated with HOST_IP in .env example

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -17,6 +17,7 @@ services:
       - LOG_LEVEL=${LOG_LEVEL:-INFO}
       - DATA_DIR=${DATA_DIR:-/opt/netbird-instances}
       - DOCKER_NETWORK=${DOCKER_NETWORK:-npm-network}
+      - HOST_IP=${HOST_IP:-}
     networks:
       - npm-network
     healthcheck: