playground: update Dockerfile to use Go 1.9.3

Change-Id: I7e11992ab20f2b268d83c1cfbe86e890c226f96f
Reviewed-on: https://go-review.googlesource.com/90615
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/Dockerfile b/Dockerfile
index 1230d52..a1ca757 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -7,7 +7,7 @@
 ENV GOPATH /go
 ENV PATH /usr/local/go/bin:$GOPATH/bin:$PATH
 ENV GOROOT_BOOTSTRAP /usr/local/gobootstrap
-ENV GO_VERSION 1.9.2
+ENV GO_VERSION 1.9.3
 ENV DEPS 'ca-certificates'
 ENV BUILD_DEPS 'curl bzip2 git gcc patch libc6-dev'