Add never option to PUBLIC_URL_DETECTION configuration (#36785)

Follow up #34250

Docs: https://gitea.com/gitea/docs/pulls/353

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Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/httplib/url.go

@@ -72,6 +72,10 @@ func GuessCurrentAppURL(ctx context.Context) string {
 
 // GuessCurrentHostURL tries to guess the current full host URL (no sub-path) by http headers, there is no trailing slash.
 func GuessCurrentHostURL(ctx context.Context) string {
+	// "never" means always trust ROOT_URL and skip any request header detection.
+	if setting.PublicURLDetection == setting.PublicURLNever {
+		return strings.TrimSuffix(setting.AppURL, setting.AppSubURL+"/")
+	}
 	// Try the best guess to get the current host URL (will be used for public URL) by http headers.
 	// At the moment, if site admin doesn't configure the proxy headers correctly, then Gitea would guess wrong.
 	// There are some cases: