playground: upgrade to Debian Buster

Buster is the latest stable release of Debian. Although Stretch is still
covered by LTS, it's better to be up to date.

Updates golang/go#25224

Change-Id: If91b7f3de834a6683880a1f661ff8777f58c38e7
Reviewed-on: https://go-review.googlesource.com/c/playground/+/227348
Reviewed-by: Carlos Amedee <[email protected]>

Author: toothrot

Dockerfile

@@ -3,14 +3,14 @@
 # license that can be found in the LICENSE file.
 
 ############################################################################
-FROM debian:stretch AS nacl
+FROM debian:buster AS nacl
 
 RUN apt-get update && apt-get install -y --no-install-recommends curl bzip2 ca-certificates
 
 RUN curl -s https://storage.googleapis.com/nativeclient-mirror/nacl/nacl_sdk/trunk.544461/naclsdk_linux.tar.bz2 | tar -xj -C /tmp --strip-components=2 pepper_67/tools/sel_ldr_x86_64
 
 ############################################################################
-FROM debian:stretch AS build
+FROM debian:buster AS build
 LABEL maintainer="[email protected]"
 
 ENV BUILD_DEPS 'curl git gcc patch libc6-dev ca-certificates'
@@ -82,7 +82,7 @@ RUN ../bin/go install --tags=faketime std
 
 ############################################################################
 # Final stage.
-FROM debian:stretch
+FROM debian:buster
 
 RUN apt-get update && apt-get install -y git ca-certificates --no-install-recommends