💚 Bake Node into the CI job image

setup-node never consults `node` on PATH; its only fast path is a probe of
/opt/hostedtoolcache, which the ephemeral act_runner job containers always
miss, so every job paid a ~50 MB Node download. docker/Dockerfile extends the
runner's default catthehacker/act image with the Node distribution overlaid at
the exact tool-cache layout, so setup-node finds 26.8.2 and skips the fetch
while node-version-file, cache: npm and registry-url keep working unchanged.

Image layers dedupe against the base the host already pulled and prune via
normal docker hygiene — the cleanup story a host bind of /opt/hostedtoolcache
lacks. To make the bake deterministic, .node-version is pinned to the exact
26.8.2 the image carries; scripts/runner-image.sh guards that coupling and
builds/pushes the tag the three node jobs now reference via container.image.

Ops follow-up (outside the repo): build once with
`npm run build:runner-image -- --push` on a machine with registry creds. If
the package is private, the runner needs container registry credentials in
its config.

docker/Dockerfile

@@ -0,0 +1,30 @@
+# CI job image for the Gitea act_runner: the runner's default job image with
+# Node pre-planted where actions/setup-node looks first.
+#
+# Why this layout: setup-node ignores `node` on PATH; its only fast path is a
+# probe of /opt/hostedtoolcache/node/<version>/<arch>. Without an entry there
+# it downloads the ~50 MB distribution on EVERY job (the runner's job
+# containers are ephemeral, so its tool cache never survives a job). The
+# official node images keep exactly the layout setup-node expects under
+# /usr/local, so this layer is a pure file overlay — no scripts, no env.
+#
+# Why not a host bind of /opt/hostedtoolcache: binds never self-prune. Docker
+# images are content-addressed: the base layers dedupe against the act image
+# the host already has, and `docker image prune` / re-pulls are the cleanup
+# story.
+#
+# NODE_VERSION must match `.node-version` exactly. setup-node resolves a float
+# like `26` to the latest known patch at runtime, so a bump silently busts the
+# baked entry; `.node-version` is pinned to x.y.z and scripts/runner-image.sh
+# guards the coupling. Rebuild + repoint `container.image` in
+# .gitea/workflows/ci.yml on every bump.
+FROM catthehacker/ubuntu:act-latest
+
+ARG NODE_VERSION=26.8.2
+
+# node image: bin/ + lib/ under /usr/local → tool cache: bin/ + lib/ under <ver>/x64.
+COPY --from=node:${NODE_VERSION} /usr/local /opt/hostedtoolcache/node/${NODE_VERSION}/x64
+
+# Fail the build (not CI) if the overlay or the version arg were wrong.
+# Shell form on purpose: exec form (`RUN [...]`) does not expand ARG values.
+RUN "/opt/hostedtoolcache/node/${NODE_VERSION}/x64/bin/node" --version