feat(catalog): build-container-buildah component + sim

Andrew Dunnrequested to merge
feat/build-container-buildah into main

First of three v2.1.0 engine components. Functionally equivalent to the legacy container component; adds explicit-engine naming for symmetry with build-container-buildkit and build-container-kaniko still to come.

What's new

  • templates/build-container-buildah/template.yml — uses ci-buildah via $[[ component.version ]]. Exposes an image input so consumers (and the sim harness) can override.
  • templates/build-container-buildah/README.md — usage, inputs, relationship to legacy container.
  • .gitlab/ci/sim/sim-buildah.yml + sim-buildah.Containerfile — sim builds a tiny UBI-micro fixture image and validates buildah + cosign on PATH and cosign sign-against-digest.

Backward compatibility

The legacy container component continues to work and is not deprecated in v2.x. v3.0.0 will retire it in favour of build-container-buildah.

Validation plan

  • MR pipeline green (syntax + lint-templates).
  • After merge: main pipeline triggers sim-buildah (relevant-file changes touched). Sim builds + signs the fixture image.
  • If sim green: build-container-buildah is proven. Continue with buildkit POC sim.
  • If sim red: triage and patch before any v2.1.0 tag.

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