feat: Add multi-architecture Docker image build script and update README

scripts/build-multiarch-image.sh

@@ -0,0 +1,79 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+usage() {
+  echo "Usage: $0 <image-tag>" >&2
+  echo "Set IMAGE_REPOSITORY to override the default repository." >&2
+  exit 2
+}
+
+if [[ $# -ne 1 ]]; then
+  usage
+fi
+
+tag=$1
+if [[ ! $tag =~ ^[A-Za-z0-9_][A-Za-z0-9_.-]{0,121}$ ]]; then
+  echo "Invalid Docker image tag: $tag" >&2
+  exit 2
+fi
+
+repository=${IMAGE_REPOSITORY:-${DOCKER_USERNAME:-dalbodeule}/dnsapi}
+publish_latest=${PUBLISH_LATEST:-1}
+dry_run=${DRY_RUN:-0}
+resume=${RESUME:-0}
+root_dir=$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)
+
+if [[ $publish_latest != 0 && $publish_latest != 1 ]]; then
+  echo "PUBLISH_LATEST must be 0 or 1" >&2
+  exit 2
+fi
+if [[ $dry_run != 0 && $dry_run != 1 ]]; then
+  echo "DRY_RUN must be 0 or 1" >&2
+  exit 2
+fi
+if [[ $resume != 0 && $resume != 1 ]]; then
+  echo "RESUME must be 0 or 1" >&2
+  exit 2
+fi
+if [[ $dry_run == 0 ]]; then
+  command -v docker >/dev/null || { echo "Docker is required" >&2; exit 1; }
+  docker buildx version >/dev/null
+fi
+
+run() {
+  if [[ $dry_run == 1 ]]; then
+    printf 'DRY RUN:'
+    printf ' %q' "$@"
+    printf '\n'
+  else
+    "$@"
+  fi
+}
+
+cd "$root_dir"
+for arch in amd64 arm64; do
+  image="$repository:$tag-$arch"
+  if [[ $resume == 1 && $dry_run == 0 ]] && docker image inspect "$image" >/dev/null 2>&1; then
+    actual_platform=$(docker image inspect "$image" --format '{{.Os}}/{{.Architecture}}')
+    if [[ $actual_platform != "linux/$arch" ]]; then
+      echo "$image has platform $actual_platform, expected linux/$arch" >&2
+      exit 1
+    fi
+    echo "Reusing local image $image"
+  else
+    run ./gradlew bootBuildImage \
+      -PmultiArchImage=true \
+      "--imagePlatform=linux/$arch" \
+      "--imageName=$image" \
+      --no-publishImage
+  fi
+  run docker push "$image"
+done
+
+manifest_args=(docker buildx imagetools create --tag "$repository:$tag")
+if [[ $publish_latest == 1 ]]; then
+  manifest_args+=(--tag "$repository:latest")
+fi
+manifest_args+=("$repository:$tag-amd64" "$repository:$tag-arm64")
+run "${manifest_args[@]}"
+run docker buildx imagetools inspect "$repository:$tag"