Dockerfile: update from jessie to stretch
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 <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/playground,golang/playground,golang/playground