Move DOCKERHUB_USERNAME from vars to secrets so merge-docker can access
credentials without its own environment scope. This eliminates the
redundant double approval since build-docker already gates on
release-publish.
Replace QEMU emulation with parallel native builds using a matrix
strategy (ubuntu-latest for amd64, ubuntu-24.04-arm for arm64).
Each platform pushes by digest, then a merge job creates the
multi-arch manifest list before signing with cosign.