build: install Node 24 so JS actions run in the job container

When this image is used as a Gitea Actions `container:`, the runner exec's
`node` inside it to run JS-based actions (actions/checkout, upload-artifact).
The image had no Node, so consuming app builds failed at the checkout step
with `exec: "node": executable file not found in $PATH`.

Pull Node 24 from NodeSource (Noble's apt nodejs is v18) and assert it in
the image sanity check.

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

README.md

@@ -16,6 +16,8 @@ here — and we don't need one.
 - Android cmdline-tools, platform, and build-tools — **baked in**, so prod
   builds don't depend on Google's download endpoint at job time
 - `git`, `curl`, `unzip`
+- Node.js 24 — so Gitea Actions JS actions (`actions/checkout`,
+  `upload-artifact`, …) run when this image is used as a job `container`
 
 Pinned versions live as `ARG`s at the top of the `Dockerfile`: