add /health endpoint

Returns 200 OK after server starts.

This greatly simplifies container deployment. For example,
Kubernetes can be configured to not route traffic to imgproxy Pods
that did not pass Readiness check against /health.

server.go

@@ -175,6 +175,12 @@ func (h *httpHandler) ServeHTTP(rw http.ResponseWriter, r *http.Request) {
 		panic(invalidSecretErr)
 	}
 
+	if r.URL.Path == "/health" {
+		rw.WriteHeader(200);
+		rw.Write([]byte("imgproxy is running"));
+		return
+	}
+
 	imgURL, procOpt, err := parsePath(r)
 	if err != nil {
 		panic(newError(404, err.Error(), "Invalid image url"))