fix: incorrect cache key

[HolgerHuo]

In 4003b03, we deprecated ProxyMode switch, making this condition unavailable. We can simply detect proxy usage by the etag variable passed in.

[n0vad3v]

Proxymode is still available on configuration load, there is a function called switchProxyMode inside LoadConfig.

func switchProxyMode() {
	matched, _ := regexp.MatchString(HttpRegexp, Config.ImgPath)
	if matched {
		// Enable proxy based on ImgPath should be deprecated in future versions
		log.Warn("Enable proxy based on ImgPath will be deprecated in future versions. Use IMG_MAP config options instead")
		ProxyMode = true
	}
}

[HolgerHuo]

Yes, but only IMG_PATH is used to turn on s ProxyMode. If we write proxy backends in multipath style, the switch won't work anymore.

[n0vad3v]

Oh, I got you, yes it's indeed a problem here, merging this PR now, thanks for you contribution!