Remove duplicate docker "scratch" directory. (#4269)
Remove duplicate docker "scratch" directory. (#4269)

Automatic merge from submit-queue.

Remove duplicate docker "scratch" directory.

We already have ISTIO_DOCKER defined in main Makefile for
that purpose.
smawson/istio,istio/istio,istio/istio,istio/istio,frankbu/istio,smawson/istio,geeknoid/istio,frankbu/istio,smawson/istio,smawson/istio,geeknoid/istio,geeknoid/istio,frankbu/istio,istio/istio,geeknoid/istio,istio/istio,smawson/istio,frankbu/istio,geeknoid/istio,frankbu/istio,frankbu/istio,smawson/istio,geeknoid/istio