feat(builder): declare multi-arch builds with ARCHES

Project architecture lists belong in makefile.builder, not .env.builder.
deb/push-deb fan out when ARCH is omitted; Docker platforms derive from
ARCHES. Legacy makefiles without ARCHES keep the old single-arch behavior.

skills/builder/scripts/publish_docker.sh

@@ -17,7 +17,8 @@ usage() {
 #   DOCKER_REGISTRY      Required (or --registry)
 #   DOCKER_REPOSITORY    Optional, default: git repository name (or --repository)
 #   IMAGE_TAG            Optional, default: version.sh --docker (or --tag)
-#   PLATFORMS            Optional, default: linux/amd64 (or --platform)
+#   PLATFORMS            Optional. Else makefile.builder ARCHES mapped to
+#                        linux/<arch>,… ; else linux/amd64 (or --platform)
 #   DOCKER_DOCKERFILE    Optional, default: Dockerfile (--file)
 #   DOCKER_CONTEXT       Optional, default: . (--context)
 #   DOCKER_BUILDER       Optional buildx builder name (--builder)
@@ -53,7 +54,7 @@ fi
 registry=${DOCKER_REGISTRY:-}
 repository=${DOCKER_REPOSITORY:-$git_repo_name}
 tag=${IMAGE_TAG:-}
-platform=${PLATFORMS:-linux/amd64}
+platform=${PLATFORMS:-}
 dockerfile=${DOCKER_DOCKERFILE:-Dockerfile}
 build_context=${DOCKER_CONTEXT:-.}
 builder=${DOCKER_BUILDER:-}
@@ -126,6 +127,29 @@ if [[ "$tag" == latest && ${ALLOW_LATEST:-0} != 1 && "$mode" == push ]]; then
   echo "Set ALLOW_LATEST=1 only when the user explicitly asked for 'latest'." >&2
   exit 3
 fi
+if [[ -z "$platform" ]]; then
+  if [[ -n "$project_root" && -f "$project_root/makefile.builder" ]]; then
+    arches=$(make -C "$project_root" -f makefile.builder -s --no-print-directory arches 2>/dev/null || true)
+    mapped=
+    for arch in $arches; do
+      case "$arch" in
+        amd64|arm64)
+          if [[ -n "$mapped" ]]; then
+            mapped="$mapped,linux/$arch"
+          else
+            mapped="linux/$arch"
+          fi
+          ;;
+        *)
+          echo "Error: unsupported ARCHES entry from makefile.builder: $arch" >&2
+          exit 2
+          ;;
+      esac
+    done
+    platform=$mapped
+  fi
+  platform=${platform:-linux/amd64}
+fi
 if [[ "$mode" == load && "$platform" == *,* ]]; then
   echo "Error: --load cannot be combined with multiple platforms: $platform" >&2
   exit 2