Updates the docker test commands

tools/gce_setup/grpc_docker.sh

@@ -957,9 +957,10 @@ grpc_cloud_prod_auth_service_account_creds_gen_ruby_cmd() {
   local test_script="/var/local/git/grpc/src/ruby/bin/interop/interop_client.rb"
   local test_script+=" --use_tls"
   local gfe_flags=$(_grpc_prod_gfe_flags)
-  local added_gfe_flags=$(_grpc_svc_acc_test_flags)
+  local added_gfe_flags=$(_grpc_default_creds_test_flags)
   local env_prefix="SSL_CERT_FILE=/cacerts/roots.pem"
-  local the_cmd="$cmd_prefix '$env_prefix ruby $test_script $gfe_flags $added_gfe_flag $@'"
+  env_prefix+=" GOOGLE_APPLICATION_CREDENTIALS=/service_account/stubbyCloudTestingTest-7dd63462c60c.json"
+  local the_cmd="$cmd_prefix '$env_prefix ruby $test_script $gfe_flags $added_gfe_flags $@'"
   echo $the_cmd
 }
 
@@ -1153,6 +1154,11 @@ _grpc_svc_acc_test_flags() {
   echo " --service_account_key_file=/service_account/stubbyCloudTestingTest-7dd63462c60c.json --oauth_scope=https://www.googleapis.com/auth/xapi.zoo"
 }
 
+# default credentials test flag
+_grpc_default_creds_test_flags() {
+  echo " --oauth_scope=https://www.googleapis.com/auth/xapi.zoo"
+}
+
 # outputs the flags passed to the gcloud auth tests
 _grpc_gce_test_flags() {
   echo " --default_service_account=155450119199-r5aaqa2vqoa9g5mv2m6s3m1l293rlmel@developer.gserviceaccount.com --oauth_scope=https://www.googleapis.com/auth/xapi.zoo"