Adds image cache images to build

[bomoko]

This PR introduces a new set of build args made available to the present build that give precise reference to the images rolled out in the last deployment.

It interrogates the current deployment list and introduces build args LAGOON_CACHE_{service name} for each service.
The service name is drawn by matching any images in the deployment with the standard structure harborbase/{project}/{environment}/{service}/....

Checklist

• Affected Issues have been mentioned in the Closing issues section
• Documentation has been written/updated
• PR title is ready for changelog and subsystem label(s) applied

Explain the details for making this change. What existing problem does the pull request solve?

Details for what issue this solves are described in the linked issue

Closing issues

closes #2918

[tobybellwood]

maybe we can get a brief para in the docs about this too
https://docs.lagoon.sh/lagoon/using-lagoon-the-basics/build-and-deploy-process#3.-build-image

[bomoko]

100% @tobybellwood - I think that once we've done some testing and tweaking on the test cluster, I'll do a writeup.

[bryangruneberg]

yes! this is nice

[tobybellwood]

We need a test, and some docs for this, ping @bomoko to follow up