Re-implement parallel matrices

[Gregory-Pereira]

Redo of #363, which got reverted in #387 .
The accessing separate env and secrets doesnt work for login, this needs to be refactored. Keeping as draft for now

[Gregory-Pereira]

Changes:

• According to the docs, attestation and SBOM support is build directly into the docker/build-push-action for any version later than v4, thus we can remove our attestation steps.
• Matrices implementation ONLY for images, and not registry auth. Each image will be built and pushed to both registries at once which should be optimized due to the caching

I want to keep the feature changes small to minimize disruptions. However when we have time I want to next investigate removing the caching step. This is because we are caching Docker layers using actions/cache@v4, but docker/build-push-action@v6 supports its own caching mechanism (type=gha). While this used to make sense for separate build steps, now that we have aggregated them, technically it shouldn't be needed.

[Gregory-Pereira]

/HOLD

Waiting for the Jan 9th release so we dont ruin our workflows. Can pick this up again then.