Add Big Bang ArgoCD Container to images.txt

A couple months ago I spent some time talking to @runyontr and @joshwolf about the ArgoCD repoServer container that has the kustomize plugins we need built into it. This image is currently not a part of Iron Bank as there seems to have been a discussion in the past with Iron Bank where they expressed not wanting to manage "custom" OSS containers and plugins. From our conversation, Big Bang mentioned they would take this up with Iron Bank to either address or begin hosting this image again.

This ticket is to serve as an update/reminder to this and to provide a link to the image that Classified Ops is currently using:

registry.dso.mil/platform-one/big-bang/apps/core/argocd:1.6.1-ib.

@matthew.kaiser and I built this container and from the Iron Bank Argo image at the time and pushed it back to the GitLab container registry and we've been using it since. This container needs a better process that will rebuild it with newer versions of ArgoCD and incorporate it as a proper Big Bang container.