Parallel push to the 3 GCR registries

cloudbuild.yaml

steps:
- name: 'gcr.io/cloud-builders/docker'
  args:
  - 'build'
  - '-t'
  - 'us.gcr.io/$PROJECT_ID/test'
  - '-t'
  - 'asia.gcr.io/$PROJECT_ID/test'
  - '-t'
  - 'eu.gcr.io/$PROJECT_ID/test'
  - '.'
  id: 'build'
- name: 'gcr.io/cloud-builders/docker'
  args: ['push', 'us.gcr.io/$PROJECT_ID/test']
  waitFor: ['build']
- name: 'gcr.io/cloud-builders/docker'
  args: ['push', 'asia.gcr.io/$PROJECT_ID/test']
  waitFor: ['build']
- name: 'gcr.io/cloud-builders/docker'
  args: ['push', 'eu.gcr.io/$PROJECT_ID/test']
  waitFor: ['build']

# Specify images to tie the build to these images.
images:
- 'us.gcr.io/$PROJECT_ID/test'
- 'asia.gcr.io/$PROJECT_ID/test'
- 'eu.gcr.io/$PROJECT_ID/test'