Add GitHub actions with Workload Identity to push the container images

[mathieu-benoit]

In:

• https://acm-workshop.alwaysupalwayson.com/whereami/prepare-container/
• and https://acm-workshop.alwaysupalwayson.com/onlineboutique/set-up-onlineboutique-git-repo/

Let's have a dedicated GitHub action in associated GH repo to copy the container images from public registry to private registry, by leveraging Workload Identity Federation:

• https://alwaysupalwayson.com/posts/2022/01/workload-identity-federation/
• https://cloud.google.com/config-connector/docs/reference/resource-docs/iam/iamworkloadidentitypool