Allow for multiple write-back targets to be defined

[Hsn723]

Is your feature request related to a problem? Please describe.

Argo CD Image Updater's git write-back configuration allows to specify the location of a single kustomization file to write back to via the argocd-image-updater.argoproj.io/write-back-target annotation, however for our use case we need to be able to write back to more than one location at once.

• Currently our ArgoCD Application manifests use Kustomize to deploy to different environments, ex: ci, staging, production
• Before deploying any image updates to staging, we'd like to test the changes in the ci environment, therefore would like to write-back to both the ci and staging kustomizations

Describe the solution you'd like Extending the argocd-image-updater.argoproj.io/write-back-target annotation to take a list of kustomization targets would resolve the use case described above. For instance:

argocd-image-updater.argoproj.io/write-back-target: "kustomization:../../ci, kustomization:../../staging"

Describe alternatives you've considered We've also considered having an instance of Argo CD Image Updater in the CI environment, however our CI runs on demand and will not pick up on changes in a timely matter. Additionally, we'd like to avoid granting additional permissions to the CI environment.

[jannfis]

Do you have different Argo CD applications configured for your ci and staging environments, or is the ci environment not connected with Argo CD at all, and just applies the kustomization to a cluster?

[Hsn723]

@jannfis All our environment use the same Argo CD Applications with different Kustomize overlays. In CI, a new instance of Argo CD is deployed each time, along with the parent Application which in turns picks up our repository's Applications in an app-of-apps pattern.