sandbox: autoscale with 5 instances as a minimum

We're underprovisioned, and the autoscaling seems to react too slowly to
an increase in traffic.

Set the minimum scaling to 5 instances, which should be enough to serve
all levels of traffic throughout the day.

Change-Id: I3faadbc7647adb2ee39e1a34f423e7449b9ba520
Reviewed-on: https://go-review.googlesource.com/46714
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

sandbox/app-flex.yaml

diff --git a/sandbox/app-flex.yaml b/sandbox/app-flex.yaml
index b9501dc..fad29f8 100644
--- a/sandbox/app-flex.yaml
+++ b/sandbox/app-flex.yaml
@@ -2,6 +2,10 @@
 runtime: custom
 env: flex
 
+automatic_scaling:
+  min_num_instances: 5
+  max_num_instances: 20
+
 health_check:
   check_interval_sec: 20
   restart_threshold: 10