💚 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.

.gitea/workflows/ci.yml

@@ -49,11 +49,16 @@ jobs:
         needs: release-gate
         if: needs.release-gate.outputs.skip != 'true'
         runs-on: ubuntu-latest
-        # Bind-mount the shared pages tree so the coverage step below can write
-        # into it. The runner whitelists this path via `container.valid_volumes`
-        # (docker-space `setup/gitea.sh`); `image` is omitted on purpose so the
-        # runner keeps using its default job image.
+        # `image` extends the runner's default job image (catthehacker/act)
+        # with Node 26 pre-planted in the tool cache layout, so setup-node's
+        # version probe hits and never downloads (see docker/Dockerfile). The
+        # tag MUST equal the exact version pinned in `.node-version`; rebuild
+        # via `npm run build:runner-image -- --push` and repoint here on every
+        # Node bump. The volume bind-mounts the shared pages tree so the
+        # coverage step below can write into it; the runner whitelists this
+        # path via `container.valid_volumes` (docker-space `setup/gitea.sh`).
         container:
+            image: gitea.e1nsnull.de/tmu/act-ci:26.8.2
             volumes:
                 - /data/gitea-pages:/data/gitea-pages
         steps:
@@ -108,6 +113,10 @@ jobs:
         if: needs.release-gate.outputs.skip != 'true'
         runs-on: ubuntu-latest
         continue-on-error: true
+        # Same baked image as `build` — without it this job re-downloads Node
+        # per run (see docker/Dockerfile).
+        container:
+            image: gitea.e1nsnull.de/tmu/act-ci:26.8.2
         steps:
             - uses: actions/checkout@v4
             - uses: actions/setup-node@v4
@@ -121,6 +130,10 @@ jobs:
         if: startsWith(gitea.ref, 'refs/tags/')
         needs: build
         runs-on: ubuntu-latest
+        # Same baked image as `build` — setup-node still owns the registry-url
+        # `.npmrc` rewrite here; only the Node download is skipped.
+        container:
+            image: gitea.e1nsnull.de/tmu/act-ci:26.8.2
         # The release page is created with the run's automatic Gitea token
         # (`github.token`), not `NPM_TOKEN`, so it needs `contents: write`.
         permissions: