freeze docker build image version

Dockerfile.npc

@@ -1,4 +1,4 @@
-FROM golang:1.13.8 as builder
+FROM golang:1.13 as builder
 ARG GOPROXY=direct
 WORKDIR /go/src/ehang.io/nps
 COPY . .

Dockerfile.nps

@@ -1,4 +1,4 @@
-FROM golang:1.13.8 as builder
+FROM golang:1.13 as builder
 ARG GOPROXY=direct
 WORKDIR /go/src/ehang.io/nps
 COPY . .