docker: update go version to 1.13 (pingcap#13021)

Author: cwen0

Dockerfile

@@ -1,5 +1,5 @@
 # Builder image
-FROM golang:1.12-alpine as builder
+FROM golang:1.13-alpine as builder
 
 RUN apk add --no-cache \
     wget \
@@ -34,4 +34,4 @@ WORKDIR /
 
 EXPOSE 4000
 
-ENTRYPOINT ["/usr/local/bin/dumb-init", "/tidb-server"]
+ENTRYPOINT ["/usr/local/bin/dumb-init", "/tidb-server"]