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>

README.md

@@ -22,10 +22,10 @@ Pinned versions live as `ARG`s at the top of the `Dockerfile`:
 | ARG                     | Default     | Where to check |
 | ----------------------- | ----------- | -------------- |
 | `CMDLINE_TOOLS_VERSION` | `13114758`  | https://developer.android.com/studio#command-line-tools-only |
-| `BUILD_TOOLS_VERSION`   | `35.0.0`    | SDK Manager / release notes |
-| `PLATFORM_VERSION`      | `android-35`| your app's `compileSdk` |
+| `BUILD_TOOLS_VERSION`   | `36.0.0`    | SDK Manager / release notes |
+| `PLATFORM_VERSION`      | `android-36`| your app's `compileSdk` |
 
-`aapt2` and the build-tools binaries ship native arm64 for 35.x, so packaging
+`aapt2` and the build-tools binaries ship native arm64 for 36.x, so packaging
 runs without emulation.
 
 ## Tagging model
@@ -61,7 +61,7 @@ app and pin the toolchain:
 ```yaml
 jobs:
   build:
-    runs-on: [self-hosted, arm64]
+    runs-on: ubuntu-24.04-arm64
     container:
       image: git.helu.ca/r/android:2026.06
       credentials: