fix(ci): add linux/arm64 Docker build support with QEMU optimization (#76)

- Add docker/setup-qemu-action@v3 to docker-publish.yml (was missing,
  causing multi-platform builds to hang indefinitely)
- Add platforms: linux/amd64,linux/arm64 to build-push-action step
- Optimize Dockerfile with --platform=$BUILDPLATFORM on builder stages
  so TypeScript compilation runs at native amd64 speed; only the lean
  npm ci --omit=dev step runs under QEMU in the final stage, compiling
  the three native modules (bcrypt, better-sqlite3, node-pty) for the
  correct target architecture — reduces arm64 build time from 6+ hours
  to ~15-30 minutes

.github/workflows/docker-publish.yml

@@ -16,8 +16,10 @@ jobs:
       - name: Check out the repo
         uses: actions/checkout@v4
 
-      - name: Set up QEMU (for ARM64 support)
+      - name: Set up QEMU
         uses: docker/setup-qemu-action@v3
+        with:
+          platforms: arm64
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
@@ -44,8 +46,8 @@ jobs:
         uses: docker/build-push-action@v5
         with:
           context: .
-          platforms: linux/amd64,linux/arm64
           push: true
+          platforms: linux/amd64,linux/arm64
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
           cache-from: type=registry,ref=saelix/sencho:buildcache