Validate an Nginx configuration file in a CI container before reload

# Run syntax test inside ephemeral Nginx alpine container:
docker run --rm -t 
    -v "$(pwd)/nginx.conf:/etc/nginx/nginx.conf:ro" 
    -v "$(pwd)/conf.d:/etc/nginx/conf.d:ro" 
    nginx:alpine nginx -t -q 
    && echo "Nginx configuration syntax is valid." 
    || { echo "Nginx syntax check failed!"; exit 1; }