Support for setting keepAliveTimeout

相關進入點是建立 httpServer 和 httpsServer 的 HTTP/HTTPS 伺服器設定；先追蹤 Echo 容器如何讀取執行階段環境變數。為兩個伺服器加入可設定的 keep-alive 逾時，包括 issue 中顯示的相關 headers-timeout 行為，並確認無論是否設定此項，容器都仍能啟動並接受 HTTP/S 請求。

Hi,

I was making some tests with the Echo container, including ensuring connection pool logics / configuration.
Currently this is taking the default 5s Node keepAliveTimeout which will then start dropping connections. If we would be able to stablish the keepAliveTimeout we would be able to allow the consumer side to manage the connections.

I've hacked a solution as:

// How long an idle keep-alive connection is held before the server closes it.
// Node's default is 5s, so an upstream keep-alive pool in front of this server can never be observed intact. 
// Raising it past the drain time lets the pool settle and leaves the reverse proxy in charge of trimming it.
const keepAliveTimeoutMs = parseInt(process.env.KEEPALIVE_TIMEOUT_MS, 10);
if (Number.isFinite(keepAliveTimeoutMs) && keepAliveTimeoutMs > 0) {
    for (const server of [httpServer, httpsServer]) {
        server.keepAliveTimeout = keepAliveTimeoutMs;
        // headersTimeout bounds how long a started request may take to send its headers; 
        // keeping it above keepAliveTimeout stops Node from closing a pooled connection just as it is being reused.
        server.headersTimeout = keepAliveTimeoutMs + 5000;
    }
    console.log(`Keep-alive timeout set to ${keepAliveTimeoutMs}ms (headers timeout ${keepAliveTimeoutMs + 5000}ms).`);
}