playground: set GOPROXY when building Docker image

Change-Id: Ia59f12bb2ac6576409d10645885c8fd87ff528b1
Reviewed-on: https://go-review.googlesource.com/c/playground/+/188240
Reviewed-by: Alexander Rakoczy <alex@golang.org>
diff --git a/Dockerfile b/Dockerfile
index 7ddad2a..0943771 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -44,6 +44,7 @@
 RUN mkdir /gocache
 ENV GOCACHE /gocache
 ENV GO111MODULE on
+ENV GOPROXY=https://proxy.golang.org
 
 COPY go.mod /go/src/playground/go.mod
 COPY go.sum /go/src/playground/go.sum