ci: wire up the builder-image workflow and fix repo layout

The repo was committed with the CI files misnamed/misplaced, so nothing
ran. Move them into place and fix the issues that surfaced against the
Syrinx reference and the Dade target:

- Dockerfile.yml -> Dockerfile (the workflow references ./Dockerfile)
- builder-image.yml -> .gitea/workflows/builder-image.yml (Gitea only
  runs files under .gitea/workflows)
- build.yml -> templates/build.yml (it's the app-repo template, not a
  workflow for this repo; keep it out of the run path)
- runs-on [self-hosted, arm64] -> ubuntu-24.04-arm64 (the real Gitea
  label, matching Syrinx) in the workflow, template, and README
- bump image SDK android-35/build-tools 35.0.0 -> android-36/36.0.0 to
  match Dade's compileSdk 36 (avoids a job-time SDK download)
- template upload-artifact@v4 -> @v3 (v4 is unsupported on Gitea)

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

.gitea/workflows/builder-image.yml

@@ -0,0 +1,57 @@
+name: build-android-builder-image
+
+# Builds the Android toolchain image and pushes it to the Gitea registry.
+# Push a tag like `2026.06` to cut an immutable toolchain; pushes to main
+# refresh the moving `latest` tag.
+on:
+  push:
+    branches: [main]
+    tags: ["*"]
+  workflow_dispatch:
+
+env:
+  REGISTRY: git.helu.ca
+  IMAGE: git.helu.ca/r/android
+
+jobs:
+  build-image:
+    runs-on: ubuntu-24.04-arm64
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      # Determine which tags to push:
+      #  - a git tag  -> use it verbatim (e.g. 2026.06) + latest
+      #  - main branch -> latest only
+      - name: Compute tags
+        id: tags
+        run: |
+          if [ "${{ gitea.ref_type }}" = "tag" ]; then
+            REF="${{ gitea.ref_name }}"
+            echo "tags=${IMAGE}:${REF},${IMAGE}:latest" >> "$GITHUB_OUTPUT"
+          else
+            echo "tags=${IMAGE}:latest" >> "$GITHUB_OUTPUT"
+          fi
+
+      - name: Log in to Gitea registry
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ gitea.actor }}
+          password: ${{ secrets.GITEA_TOKEN }}
+
+      - name: Set up Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Build and push (linux/arm64)
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          file: ./Dockerfile
+          platforms: linux/arm64
+          push: true
+          tags: ${{ steps.tags.outputs.tags }}
+          # Bump these here if you want to override the Dockerfile defaults
+          # build-args: |
+          #   BUILD_TOOLS_VERSION=35.0.0
+          #   PLATFORM_VERSION=android-35