Build a nodejs app with 2 different nodejs versions

cloudbuild.yaml

steps:
- name: 'gcr.io/cloud-builders/docker'
  args: ['build', '--build-arg', 'node_version=${_NODE_VERSION_1}', '-t', 'gcr.io/$PROJECT_ID/build-substitutions-nodejs-${_NODE_VERSION_1}', '.']
  wait_for: '-'
- name: 'gcr.io/cloud-builders/docker'
  args: ['build', '--build-arg', 'node_version=${_NODE_VERSION_2}', '-t', 'gcr.io/$PROJECT_ID/build-substitutions-nodejs-${_NODE_VERSION_2}', '.']
  wait_for: '-'

images: ['gcr.io/$PROJECT_ID/build-substitutions-nodejs-${_NODE_VERSION_1}', 'gcr.io/$PROJECT_ID/build-substitutions-nodejs-${_NODE_VERSION_2}']

substitutions: # default versions
  _NODE_VERSION_1: v6.9.5
  _NODE_VERSION_2: v7.7.3

options:
  substitution_option: 'ALLOW_LOOSE' # meaning the build won't return an error if there is a missing substitution variable, or a missing substitution