Update Go version to 1.13

Vighneswar Rao Bojja · bvighnesha · commit ba7f611e9f54 · 2019-10-04T14:51:03.000+05:30
diff --git a/Dockerfile b/Dockerfile
@@ -1,4 +1,4 @@
-FROM golang:1.12 as builder
+FROM golang:1.13 as builder
 ARG VERSION
 ARG GIT_COMMIT
 WORKDIR /go/src/github.com/nginxinc/nginx-prometheus-exporter

Original file line number	Diff line number	Diff line change
`@@ -1,4 +1,4 @@`
`1`		`-FROM golang:1.12 as builder`
	`1`	`+FROM golang:1.13 as builder`
`2`	`2`	`ARG VERSION`
`3`	`3`	`ARG GIT_COMMIT`
`4`	`4`	`WORKDIR /go/src/github.com/nginxinc/nginx-prometheus-exporter`