Use ETag caching when running from source

[alexdima]

Follow up from #147311 (comment)

Doing the following steps:

• scripts/code-server.sh
• open the web ui
• uncheck "Disable caching" in DevTools
• cause a page load by pressing Enter in the URL bar (cmd+r sometimes is interpreted by some browsers as load without cache, so use that if your browser doesn't disable caching when pressing it)

Without ETag (59.5MB transferred, page load 3.82s):

With ETag (275kB transferred, page load 2.83s)

• apply the patch:

diff --git a/src/vs/server/node/webClientServer.ts b/src/vs/server/node/webClientServer.ts
index 6053e046a51..11e361ee77d 100644
--- a/src/vs/server/node/webClientServer.ts
+++ b/src/vs/server/node/webClientServer.ts
@@ -158,7 +158,7 @@ export class WebClientServer {
 			return serveError(req, res, 400, `Bad request.`);
 		}
 
-		return serveFile(filePath, this._environmentService.isBuilt ? CacheControl.NO_EXPIRY : CacheControl.NO_CACHING, this._logService, req, res, headers);
+		return serveFile(filePath, this._environmentService.isBuilt ? CacheControl.NO_EXPIRY : CacheControl.ETAG, this._logService, req, res, headers);
 	}
 
 	private _getResourceURLTemplateAuthority(uri: URI): string | undefined {

Do the same steps:

This pays off especially when using a codespace to develop vscode web, etc.

[aeschli]

Ok, convinced, thanks for the measurements!