docker - How do I set an environment or substitution variable via a step in Google Cloud Build?

Question

Basically, when using Google Cloud Build, how do I read a value that was written in an earlier build step in subsequent steps?

Specifically, I'd like to make a custom image tag that's based on a combination of the timestamp and $SHORT_SHA. Something like the below. Though, it doesn't work, as docker complains about "export", and, even if that worked, it likely will be a different env:

  # Setting tag in a variable:
  - name: 'ubuntu'
    args: ['export', '_BUILD_TAG=`date', '-u', '+%Y%m%dT%H%M%S_$SHORT_SHA`']

Then, in a later step:

  # Using tag from the variable:
  - name: gcr.io/cloud-builders/docker
    args: ['build', '-t', 'gcr.io/$PROJECT_ID/$_BUILD_TAG', '.']

So, how do I use the output of one step in another? I could write the contents of date to a file, and then read it, but I'm back at not knowing how to set the variable from the file I read (or otherwise interpolate its results to form the argument to docker build).

Answer 1

Waitting for answers