devapp/devappserver: add Dockerfile in preparation for running on GKE

Since we can’t run an App Engine Flex app on *.golang.org (see
issue), the Flex app.yaml file is removed.

Updates golang/go#20691

Change-Id: Id775962349a88b119c9fe671f9a5180f70528836
Reviewed-on: https://go-review.googlesource.com/46133
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
5 files changed