Dockerfile: update from jessie to stretch

jessie is old (even by Debian standards) and doesn't even build
anymore without changes, as the apt-get contents have moved to an
archive URL.

Debian stable is stretch these days, so upgrade to that. (Debian buster
is almost the next stable, but not quite yet)

Change-Id: I633edc56d92cea37c7ed01f6e85b4bea4ff3b412
Reviewed-on: https://go-review.googlesource.com/c/playground/+/176277
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: bradfitz

Dockerfile

@@ -1,7 +1,7 @@
 # Copyright 2017 The Go Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.
-FROM debian:jessie as builder
+FROM debian:stretch as builder
 LABEL maintainer "[email protected]"
 
 ENV GOPATH /go
@@ -215,7 +215,7 @@ RUN go install cloud.google.com/go/compute/metadata \
 COPY . /go/src/playground/
 RUN go install playground
 
-FROM debian:jessie
+FROM debian:stretch
 
 RUN apt-get update && apt-get install -y git ca-certificates --no-install-recommends