How to pass context in golang request to middleware

I am trying to understand how the context introduced in Golang 1.7 works and what would be the appropriate way to pass it to middleware and to a HandlerFunc. Should the context get initialized in the main function and passed to the checkAuth function then? And how to pass it to Hanlder and the ServeHTTP function? I read Go concurrency patterns and How to use Context but I struggle to adapt those patterns to my code.

func checkAuth(authToken string) util.Middleware {
    return func(h http.Handler) http.Handler {
        return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
            if r.Header.Get("Auth") != authToken {
                util.SendError(w, "...", http.StatusForbidden, false)
                return
            }
            h.ServeHTTP(w, r)
        })
    }
}

// Handler is a struct
type Handler struct {
    ...
    ...
}

// ServeHTTP is the handler response to an HTTP request
func (h *HandlerW) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    decoder := json.NewDecoder(r.Body)

    // decode request / context and get params
    var p params
    err := decoder.Decode(&p)
    if err != nil {
       ...
        return
    }

    // perform GET request and pass context
    ...


}


func main() {
    router := mux.NewRouter()

    // How to pass context to authCheck?
    authToken, ok := getAuthToken()
    if !ok {
        panic("...")
    }
    authCheck := checkAuth(authToken)

    // initialize middleware handlers
    h := Handler{
       ...
   } 

   // chain middleware handlers and pass context
   router.Handle("/hello", util.UseMiddleware(authCheck, Handler, ...))
}