playground: enable third-party imports

Updates golang/go#31944

Change-Id: Iffc0755cca419b72d8facd8ece950e78cdae892e
Reviewed-on: https://go-review.googlesource.com/c/playground/+/176940
Reviewed-by: Andrew Bonventre <andybons@golang.org>

Author: bradfitz

Dockerfile

@@ -94,7 +94,7 @@ WORKDIR /app
 RUN /app/playground test
 
 # Whether we allow third-party imports via proxy.golang.org:
-ENV ALLOW_PLAY_MODULE_DOWNLOADS false
+ENV ALLOW_PLAY_MODULE_DOWNLOADS true
 
 EXPOSE 8080
 ENTRYPOINT ["/app/playground"]