Make sure you have Terraform and gcloud CLI installed on your machine and authenticated to your Google Cloud Project.
Then Populate variables inside variables.auto.tfvars and run:
terraform init
terraform validate
terraform plan -out tfplan
terraform apply "tfplan"
gcloud builds submit --no-source --config=build.yml
When this command finishes execution, You will see the IP address of the Proxy Server printed at the end which indicates that the outside world thinks Cloud Build Instances have that IP address.