install-docker.sh: use registry mirror mirror.gcr.io

install-docker.sh

-#!/bin/sh
+#!/bin/bash
 #
 # This script helps us to prepare a Docker host for the build system
 #
@@ -12,6 +12,14 @@ run() {
   (set -x; "$@")
 }
 
+sudo mkdir -p /etc/docker
+
+cat <<EOF |sudo tee /etc/docker/daemon.json >/dev/null
+{
+  "registry-mirrors": ["https://mirror.gcr.io"]
+}
+EOF
+
 echo "Installing Docker via get.docker.com"
 run curl -LsS https://get.docker.com -o /tmp/get-docker.sh
 run sh /tmp/get-docker.sh