ci(publish): build container images on native arch runners

Each distro now builds amd64 on ubuntu-latest and arm64 on
ubuntu-24.04-arm instead of emulating arm64 under QEMU. image.sh gains
--platform, which pushes one <version>-<arch> tag, and --merge, which
joins those tags under the release, stable and alias tags with
buildx imagetools. The workflow splits into prepare, build and merge
jobs; scripts/build/publish.sh follows the same sequence locally.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

scripts/github/publish-containers/merge-container-images.sh

@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+: "${OWNER:?OWNER must be set}"
+: "${VERSION:?VERSION must be set}"
+: "${IS_STABLE:?IS_STABLE must be set}"
+: "${PKGMGR_DISTRO:?PKGMGR_DISTRO must be set}"
+
+bash scripts/build/image.sh \
+  --publish \
+  --registry ghcr.io \
+  --owner "${OWNER}" \
+  --version "${VERSION}" \
+  --stable "${IS_STABLE}" \
+  --merge

scripts/github/publish-containers/publish-container-images.sh

@@ -3,6 +3,12 @@ set -euo pipefail
 
 : "${OWNER:?OWNER must be set}"
 : "${VERSION:?VERSION must be set}"
-: "${IS_STABLE:?IS_STABLE must be set}"
+: "${PKGMGR_DISTRO:?PKGMGR_DISTRO must be set}"
+: "${PLATFORM:?PLATFORM must be set}"
 
-bash scripts/build/publish.sh
+bash scripts/build/image.sh \
+  --publish \
+  --registry ghcr.io \
+  --owner "${OWNER}" \
+  --version "${VERSION}" \
+  --platform "${PLATFORM}"