Add porter build cache

[tamirkamara]

What is being addressed

With a recent upgrade of Porter we can introduce build cache much like we do with the application images we build. This will help with the following:

1. dockerhub pull throttling - since we use anonymous pulls from docker hub it's limited and we sometimes see errors in CI bundle builds in this repo. Caching will reduce this issue from appearing.
2. The actual build time is reduced by 1-2 mins. On its own not that important but anything helps.

[github-actions]

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[Copilot]

Pull request overview

This PR introduces a Docker/ACR-backed build cache for Porter bundle builds to reduce Docker Hub pulls and improve CI build times.

Changes:

• Extend bundle-build in the Makefile to log into an optional CI cache ACR and export its FQDN for downstream scripts.
• Update devops/scripts/porter_build_bundle.sh to configure a Docker buildx builder and pass registry cache parameters to porter build when CI cache ACR is configured.
• Document the enhancement in CHANGELOG.md under ENHANCEMENTS.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
devops/scripts/porter_build_bundle.sh	Adds conditional Docker buildx builder setup and registry-based build cache configuration for Porter bundle builds.
Makefile	Wires CI cache ACR into the bundle build flow by logging in and exporting CI_CACHE_ACR_FQDN before calling the Porter build script.
CHANGELOG.md	Records the addition of Porter build cache in CI as an enhancement.