Update Agro Guestbook sample apps to support arm platform 

ArgoCD example apps crash with the following error when setting up Argo in Minikube on Apple silicon.

```zsh
exec /usr/local/bin/docker-php-entrypoint: exec format error
```

It seems that the guestbook UI image does not support the `arm64` platform. Would be great if we could update the examples to support arm platform as well.