Add 'latest' image tag for easier manual deployment

[marcomicera]

Quartermaster can be deployed both with GitHub Actions and manually.

Manual deployment requires developers to specify Docker image tags in the QMSTR kustomization.yaml file, as the GitHub Actions workflow does not tag those images with latest and the job.yaml doesn't specify any tags at all (kustomize takes care of adding those).

Proposed solution

This step could be made optional if we include the latest tag in the job.yaml file and let the GitHub Actions workflow add such a tag to all images.

Developers would still be able to manually set those tags in kustomization.yaml and the GitHub Actions workflow would still be deploying sha-tagged images.