tidy post deploy & import externaldns container

[khowling]

PR Summary

• Tidy up the post-deploy script, with comments
• If using external-dns with private cluster, import the external-dns image to the ACR

PR Checklist

• PR has a meaningful title
• Summarized changes
• This PR is ready to merge and is not Work in Progress
• Link to a filed issue

[khowling]

this means dont need to whitelist k8s.gcr.io in the firewall, I should really to the same with cert-manager & quay.io in a later PR @Gordonby

[Gordonby]

Looks ok.

Think we need to move to using helm charts instead of string splitting for future.

Have a look at the two relevant ones i created. They are used in the GitHub actions already.
https://github.com/Gordonby/minihelm/tree/main/samples
Perhaps we can add a backlog item to shift to helm.

[Gordonby]

LGTM