Dockerfile: update Go version to 1.13

Change-Id: I7a4dd7168e8ae8f4898de615dc5564f76114038f
Reviewed-on: https://go-review.googlesource.com/c/playground/+/192917
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>
diff --git a/Dockerfile b/Dockerfile
index 0943771..2e84a6c 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -17,8 +17,8 @@
 ENV GOPATH /go
 ENV PATH /usr/local/go/bin:$GOPATH/bin:$PATH
 ENV GOROOT_BOOTSTRAP /usr/local/gobootstrap
-ENV GO_BOOTSTRAP_VERSION go1.12.6
-ARG GO_VERSION=go1.12.6
+ENV GO_BOOTSTRAP_VERSION go1.13
+ARG GO_VERSION=go1.13
 ENV GO_VERSION ${GO_VERSION}
 
 # Fake time