为什么authorization.Request.User字符串为空？

I am working on an authorization plugin in Go and I am stuck with it. When I send commands to the Docker Engine on localhost, I get the RequestURI and RequestBody well but not the User who sent the request. It's an empty string.

func (e AuthzEntity) AuthZReq(req authorization.Request) authorization.Response {
    log.Println("RequestURI: ", req.RequestURI, " RequestBody: ", string(req.RequestBody))

    if "USER" == req.User {
        log.Println("Authorized")
    } else {
        log.Println("Unauthorized")
    }

    return authorization.Response{true, "", ""}
}

With the above code I expect the text "Authorized" in the logs, but I see "Unauthorized" and the Username is empty.

Can someone help me?