📝 Cancel force-pull task and document the runner-image decision

The act-ci image is rebuilt only when Node is bumped, which changes the
tag, so the runner's default no-force-pull behavior already picks up every
normal update. Forcing a pull would re-pull on every job and a missed
runner-side image change is acceptable, so document that decision as a
known issue with a manual docker rmi workaround instead of tracking a
force-pull task.

CONTRIBUTING.md

@@ -92,7 +92,7 @@ This is why every test in the suite pairs an `expectTypeOf(...)` with an `assert
 
 ## CI runner image
 
-The `build` / `maintain` / `publish` jobs run in `gitea.e1nsnull.de/tmu/act-ci:<version>` ([docker/Dockerfile](./docker/Dockerfile)) — the runner's default act image with the Node distribution overlaid at the exact `/opt/hostedtoolcache` layout `actions/setup-node` probes before downloading, so no job pays the ~50 MB fetch. The image tag MUST equal the exact version pinned in `.node-version`; `release-gate` uses no Node and stays on the default image. The script is deliberately NOT an `npm run` script: building requires a docker daemon and registry credentials, so it belongs to no feedback tier — per [Script prefix convention](#script-prefix-convention), no existing prefix fits and that is the signal.
+The `build` / `maintain` / `publish` jobs run in `gitea.e1nsnull.de/tmu/act-ci:<version>` ([docker/Dockerfile](./docker/Dockerfile)) — the runner's default act image with the Node distribution overlaid at the exact `/opt/hostedtoolcache` layout `actions/setup-node` probes before downloading, so no job pays the ~50 MB fetch. The image tag MUST equal the exact version pinned in `.node-version`, and the image is rebuilt only as part of a Node bump — there is no other trigger. `release-gate` uses no Node and stays on the default image. The script is deliberately NOT an `npm run` script: building requires a docker daemon and registry credentials, so it belongs to no feedback tier — per [Script prefix convention](#script-prefix-convention), no existing prefix fits and that is the signal.
 
 Bumping Node is one coordinated change, committed as a unit:
 
@@ -105,7 +105,7 @@ Skipping step 2 fails CI at image pull; skipping step 3 silently reverts to the
 Two invariants the image must satisfy for the probe to hit, both easy to break:
 
 - **The `x64.complete` marker.** `actions/tool-cache` accepts a cached tool only when `<version>/<arch>.complete` exists next to the directory (`tc.find()` checks it); a plausible-looking `node/<version>/x64/` alone is ignored and the download happens anyway. See the comment in [docker/Dockerfile](./docker/Dockerfile).
-- **Tag freshness.** The tag encodes only the Node version, so a Dockerfile change (like the marker above) produces _new content under an unchanged tag_. `act_runner` skips the pull when a tag of that name already exists locally (`forcePull=false` in the job log), so the runner must either force-pull (`force_pull` under `container:` in its `config.yaml`, if the installed version has it) or have the tag removed on the runner host (`docker rmi gitea.e1nsnull.de/tmu/act-ci:<version>`) after any image change. Symptom of getting this wrong: CI keeps running the previous image while the registry shows the new digest.
+- **Tag freshness (force-pull deliberately off).** The tag encodes only the Node version, and the image is rebuilt only when that version changes — so the normal flow always yields a new tag, and `act_runner` pulls it. `force_pull` stays disabled (the job log shows `forcePull=false`): it is acceptable to miss a runner-side image change, and forcing a pull would re-pull the image on every job for no benefit. Known issue: a Dockerfile-only change (like the marker above) re-pushed under an unchanged tag is invisible to the runner — it keeps the old image while the registry shows the new digest. If that ever matters, remove the stale tag on the runner host (`docker rmi gitea.e1nsnull.de/tmu/act-ci:<version>`); do not reach for force-pull.
 
 ## Publishing workflow
 

backlog.tasks

@@ -53,8 +53,8 @@ Maintenance:
     ✔ Write the `<version>/x64.complete` marker — actions/tool-cache ignores a bare directory, so the probe missed and the download continued @done
     ✔ Log tool-cache state from the job container to find it (temporary, removed once understood) @done
     ✔ Guard the invariant in CI (`Assert the baked tool cache is present`) @done
-☐ Enable force-pull for the runner so a changed act-ci image is never missed @low
+✘ Enable force-pull for the runner so a changed act-ci image is never missed @low @cancelled
-    → the tag encodes only the Node version, so a Dockerfile change yields new content under an unchanged tag; with `forcePull=false` the runner keeps the old image (see CONTRIBUTING § CI runner image)
+    → decided against: it is acceptable to miss a runner-side image change, and the image is only rebuilt on a Node bump, which changes the tag anyway. A Dockerfile-only change re-pushed under an unchanged tag is a known issue with a manual `docker rmi` workaround (see CONTRIBUTING § CI runner image).
 ✔ Improve CI publish @done
     ✔ Check whether publish job is only run on tags, if not, guard it @done
     ✔ Gate only single steps @done