fix: avoid QEMU arm64 crash by pinning builder stage to native arch

The pnpm install step crashes under QEMU arm64 emulation with
"Illegal instruction". Since the build output is just static
HTML/CSS/JS, pin the builder stage to $BUILDPLATFORM (amd64 on
GitHub Actions) and only cross-compile the nginx stage which
just copies files.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Copexit

Dockerfile.umbrel

@@ -1,5 +1,7 @@
-# Stage 1: Build the static export
-FROM node:22-alpine AS builder
+# Stage 1: Build the static export (always on native amd64 - output is
+# just HTML/CSS/JS so it doesn't need to match the target platform).
+# This avoids QEMU arm64 emulation crashes during pnpm install.
+FROM --platform=$BUILDPLATFORM node:22-alpine AS builder
 
 RUN corepack enable && corepack prepare pnpm@10 --activate