Feature request: registry-side tagging for unchanged images in CI

[MarekPikula]

When using Buildah in CI pipelines with cached layers stored in the same registry as the target image, there are often cases where the resulting image is identical to a previously built one (no changes in layers).

In such cases, Buildah currently:

1. Pulls all cache layers from the registry.
2. Pushes them back unchanged (effectively no-op, since the layers are already present).

This behavior results in unnecessary time and bandwidth consumption, especially for large images.

Example CI job demonstrating this behavior: https://gitlab.freedesktop.org/MarekPikula/ci-multiplatform/-/jobs/83769470

Proposed improvement:

It would be highly beneficial if Buildah could detect this scenario and skip pulling/pushing unchanged layers, instead simply tagging the existing image in the registry. This seems to be the behavior of docker buildx, and bringing a similar optimization to Buildah would significantly speed up CI workflows and reduce network usage.

I saw this briefly mentioned in #3382, but I don’t think the specific use case of skipping redundant pull/push cycles for unchanged images has been fully addressed.