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/ci-runners.compose.yml

@@ -25,11 +25,16 @@ services:
       # act-compatible runners — bundles Node, Python, git, common build
       # tools, and the Docker CLI.
       GITEA_RUNNER_LABELS: self-hosted:docker://catthehacker/ubuntu:act-22.04,on-prem:docker://catthehacker/ubuntu:act-22.04
+      # Path of the shared act_runner config file inside the container,
+      # mounted read-only via volumes below (see infra/runner-config.yaml).
+      CONFIG_FILE: /etc/runner/config.yaml
     volumes:
       # Docker socket — see security note in infra/README.md.
       - /var/run/docker.sock:/var/run/docker.sock
       # Persistent state: runner credentials live here after first registration.
       - ./data/runner-1:/data
+      # Shared act_runner configuration — see infra/runner-config.yaml.
+      - ./runner-config.yaml:/etc/runner/config.yaml:ro
     networks:
       - act-runners
 
@@ -48,9 +53,13 @@ services:
       # act-compatible runners — bundles Node, Python, git, common build
       # tools, and the Docker CLI.
       GITEA_RUNNER_LABELS: self-hosted:docker://catthehacker/ubuntu:act-22.04,on-prem:docker://catthehacker/ubuntu:act-22.04
+      # Path of the shared act_runner config file inside the container,
+      # mounted read-only via volumes below (see infra/runner-config.yaml).
+      CONFIG_FILE: /etc/runner/config.yaml
     volumes:
       - /var/run/docker.sock:/var/run/docker.sock
       - ./data/runner-2:/data
+      - ./runner-config.yaml:/etc/runner/config.yaml:ro
     networks:
       - act-runners
 
@@ -69,9 +78,13 @@ services:
       # act-compatible runners — bundles Node, Python, git, common build
       # tools, and the Docker CLI.
       GITEA_RUNNER_LABELS: self-hosted:docker://catthehacker/ubuntu:act-22.04,on-prem:docker://catthehacker/ubuntu:act-22.04
+      # Path of the shared act_runner config file inside the container,
+      # mounted read-only via volumes below (see infra/runner-config.yaml).
+      CONFIG_FILE: /etc/runner/config.yaml
     volumes:
       - /var/run/docker.sock:/var/run/docker.sock
       - ./data/runner-3:/data
+      - ./runner-config.yaml:/etc/runner/config.yaml:ro
     networks:
       - act-runners