chore(infra): pin act_runner image pull policy (#10)

## Summary
`act_runner`'s default `container.force_pull: true` re-issues a `docker pull` at the start of every job, adding 10–30 s of registry round-trip even when every layer is already locally cached. With job images pinned to specific tags (`catthehacker/ubuntu:act-22.04` and `:full-22.04`), the implicit pull is pure overhead and contradicts the deliberate-upgrade policy ADR-0015 spells out for the runner image.

- Add `infra/runner-config.yaml` with `container.force_pull: false`.
- Mount it read-only into all three runners and point each at it via `CONFIG_FILE=/etc/runner/config.yaml`.
- Document the pre-pull procedure and image-upgrade playbook in `infra/README.md` → "Job image pinning and pre-pull".
- Fold the pre-pull into the "First-time registration" walkthrough so a fresh setup is correct end-to-end.

The trade-off: the runner host must hold the images locally before the runner is asked to use them. Documented.

## Roll-out (manual, on the runner host)
```bash
cd infra/

# 1. Pre-pull the job images (one-shot — pays the cold cost once).
docker pull catthehacker/ubuntu:act-22.04
docker pull catthehacker/ubuntu:full-22.04

# 2. Recreate the runners so the new mount + env var take effect.
docker compose -f ci-runners.compose.yml up -d --force-recreate

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #10

infra/README.md

@@ -5,6 +5,7 @@ Infrastructure-as-code artefacts for the project. Separate from application code
 | Subject                                | File / Folder                                      | ADR / Reference                                                     |
 | -------------------------------------- | -------------------------------------------------- | ------------------------------------------------------------------- |
 | Self-hosted CI runners (Gitea Actions) | [`ci-runners.compose.yml`](ci-runners.compose.yml) | [ADR-0015 §"Runners"](../docs/decisions/0015-cicd-gitea-actions.md) |
+| Shared `act_runner` configuration      | [`runner-config.yaml`](runner-config.yaml)         | [ADR-0015 §"Runners"](../docs/decisions/0015-cicd-gitea-actions.md) |
 | Runtime state of the runners           | `data/` (git-ignored after `.gitignore`)           | —                                                                   |
 | Env-vars template for the runners      | `.env.example` (`.env` is git-ignored)             | —                                                                   |
 
@@ -35,10 +36,15 @@ $EDITOR .env
 #    Set GITEA_INSTANCE_URL (https, no trailing slash) and
 #    GITEA_RUNNER_REGISTRATION_TOKEN.
 
-# 3. Bring the runners up.
+# 3. Pre-pull the job images so the runner doesn't have to (see
+#    "Job image pinning and pre-pull" below for the rationale).
+docker pull catthehacker/ubuntu:act-22.04
+docker pull catthehacker/ubuntu:full-22.04
+
+# 4. Bring the runners up.
 docker compose -f ci-runners.compose.yml up -d
 
-# 4. Verify in Gitea: the three runners appear as online with the
+# 5. Verify in Gitea: the three runners appear as online with the
 #    self-hosted, on-prem labels. If a runner doesn't come online,
 #    inspect its logs:
 docker compose -f ci-runners.compose.yml logs runner-1
@@ -75,7 +81,7 @@ When this is reactivated, the right fix is one of:
 
 Either path needs a single-runner spike before being rolled out to the three.
 
-### Image pinning
+### `act_runner` image pinning
 
 The compose pins `gitea/act_runner:0.2.13`. Update the pin deliberately, not via `:latest`:
 
@@ -86,6 +92,33 @@ The compose pins `gitea/act_runner:0.2.13`. Update the pin deliberately, not via
 
 The matching CI workflows refer to runner _labels_ (not images), so a runner-image upgrade does not affect `.gitea/workflows/*`.
 
+### Job image pinning and pre-pull
+
+`act_runner` runs each job inside a container whose image is selected by the runner's _labels_. Two images are in use:
+
+| Label                  | Image                            | Used by                            |
+| ---------------------- | -------------------------------- | ---------------------------------- |
+| `self-hosted`          | `catthehacker/ubuntu:act-22.04`  | `check`, `scan`, `commits`, `a11y` |
+| `on-prem`              | `catthehacker/ubuntu:act-22.04`  | (alias of `self-hosted`)           |
+| (per-job `container:`) | `catthehacker/ubuntu:full-22.04` | `perf` (Lighthouse needs Chrome)   |
+
+[`runner-config.yaml`](runner-config.yaml) sets `container.force_pull: false`. Without that, act_runner re-issues a `docker pull` at the start of every single job (~10–30 s of registry round-trip even when every layer is already cached), which both wastes wall-clock and contradicts our policy of upgrading job images deliberately rather than implicitly via `:latest`.
+
+The trade-off: the host Docker daemon must already hold the images locally. Pre-pull them once after a fresh runner host install:
+
+```bash
+docker pull catthehacker/ubuntu:act-22.04
+docker pull catthehacker/ubuntu:full-22.04
+```
+
+Upgrading to a newer tag is a deliberate three-step process:
+
+1. Edit `GITEA_RUNNER_LABELS` (in [`ci-runners.compose.yml`](ci-runners.compose.yml)) and / or the per-job `container.image:` (in `.gitea/workflows/*`) to the new tag.
+2. On the runner host, `docker pull <new-tag>` so the image is locally available before the next CI job starts.
+3. Commit on a feature branch with a `chore(deps):` Conventional Commits subject; one of `chore(deps): upgrade CI job image to ...`.
+
+Old, no-longer-referenced images can be reaped during the periodic `docker system prune -af` (see "Disk pressure" above).
+
 ---
 
 ## Future infra concerns — placeholders