build: target amd64 — Google ships no arm64 Linux build-tools

The image pinned build-tools 36.0.0, whose aapt2/d8 are x86_64-only (Google
publishes no arm64 Linux build-tools). Built for and run on arm64, packaging
died with `aapt2: Syntax error: newline unexpected` (an x86_64 binary the
kernel can't exec).

Build the image for linux/amd64 on an amd64 runner, and point the template +
README at ubuntu-24.04. The APK output is architecture-neutral, so this costs
nothing downstream. Also execute aapt2 in the image sanity check so an arch
mismatch fails the image build, not every app build.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/builder-image.yml

@@ -15,7 +15,9 @@ env:
 
 jobs:
   build-image:
-    runs-on: ubuntu-24.04-arm64
+    # amd64: Google ships the Linux build-tools (aapt2, d8) for x86_64 only,
+    # so the toolchain must be built for and run on amd64.
+    runs-on: ubuntu-24.04
     steps:
       - name: Checkout
         uses: actions/checkout@v4
@@ -43,12 +45,12 @@ jobs:
       - name: Set up Buildx
         uses: docker/setup-buildx-action@v3
 
-      - name: Build and push (linux/arm64)
+      - name: Build and push (linux/amd64)
         uses: docker/build-push-action@v6
         with:
           context: .
           file: ./Dockerfile
-          platforms: linux/arm64
+          platforms: linux/amd64
           push: true
           tags: ${{ steps.tags.outputs.tags }}
           # Bump these here if you want to override the Dockerfile defaults