配置案例：

server {
    listen       443 ssl;
    server_name  cf-landray.yaoqinqin.cn;
    ssl_certificate      /usr/local/nginx/ssl/yaoqinqin.cn.pem;
    ssl_certificate_key  /usr/local/nginx/ssl/yaoqinqin.cn.key;
    ssl_protocols       TLSv1.2 TLSv1.3;
    ssl_ciphers         HIGH:!aNULL:!MD5;

    location / {
        proxy_pass http://127.0.0.1:8080;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Proto $scheme;

        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "upgrade";

        # 3秒判定：连不上 或 连上但不响应，都算"未启动"
        proxy_connect_timeout 3s;
        proxy_send_timeout 3s;
        proxy_read_timeout 3s;

        proxy_intercept_errors on;
        error_page 502 503 504 = @backend_down;
    }

    location @backend_down {
        default_type application/json;
        return 503 '{"code":503,"message":"后端服务尚未启动"}';
    }
}

server {
    listen       80;
    server_name  cf-landray.yaoqinqin.cn;
    return 301 https://$host$request_uri;
}