Overlay testing:

kubectl kustomize base
kubectl kustomize overlays/dev
kubectl kustomize overlays/stage

Updating image tags:

Dev

pushd overlays/dev
    kustomize edit set image \
        fow-backend-for-frontend=europe-west2-docker.pkg.dev/fow-dev-1234/docker/fow-backend-for-frontend:3e7fac648cb8e4c1f5eeb2a1dc294dbc9471fb64
popd

pushd overlays/dev
    kustomize edit set image \
        fow-be=europe-west2-docker.pkg.dev/fow-dev-1234/docker/fow-be:1c5853be0709507e2be84d25e028ede9e7fa3021
popd

Stage

pushd overlays/stage
    kustomize edit set image \
        fow-backend-for-frontend=europe-west2-docker.pkg.dev/fow-dev-1234/docker/fow-backend-for-frontend:3e7fac648cb8e4c1f5eeb2a1dc294dbc9471fb64
popd

pushd overlays/stage
    kustomize edit set image \
        fow-be=europe-west2-docker.pkg.dev/fow-dev-1234/docker/fow-be:1c5853be0709507e2be84d25e028ede9e7fa3021
popd

Unfortunately, kustomize does not allow specifying path to a kustomization and you need to cd into the folder in which you want to update image tags. See this issue for more details.

Promoting images from dev to stage

dev_images=$(yq '.images' overlays/dev/kustomization.yaml) \
    yq -i '.images = env(dev_images)' overlays/stage/kustomization.yaml

Docs:

https://github.com/kubernetes-sigs/kustomize/tree/master/examples/transformerconfigs#images-transformer

https://github.com/kubernetes-sigs/kustomize/blob/master/examples/image.md