fix(docker): build app on native build platform

docker/Dockerfile

@@ -1,4 +1,7 @@
-FROM node:22-alpine AS build
+# The Nx builds below emit platform-independent JS/static assets. During
+# multi-arch publish builds, keep that heavy build stage on the native builder
+# platform instead of running Node/Nx through QEMU for linux/arm64.
+FROM --platform=$BUILDPLATFORM node:22-alpine AS build
 
 RUN apk add --no-cache python3 make g++ git