Document in-cluster image-build CI evidence for ADR-004 runner

Records successful forgejo-actions-probe image-build workflow and the
static docker-cli + git-clone pattern for non-root host runners.

docs/forgejo-actions-runner-substrate.md

@@ -66,7 +66,13 @@ ssh coulombcore 'sudo systemctl disable --now forgejo-runner'
 ## Evidence
 
 - Host probe (2026-07-03): `coulomb/forgejo-actions-probe` workflow `success`.
-- In-cluster: verify with `make forgejo-runner-status` and a container `runs-on: ubuntu-latest` job.
+- In-cluster (2026-07-03): `railiance01-build-01` 2/2 Ready; `container-probe`
+  and `image-build` workflows `success`. Image-build uses static docker-cli on
+  the non-root runner host + `DOCKER_HOST=tcp://127.0.0.1:2375` (DinD); org
+  secrets `REGISTRY_USER` / `REGISTRY_TOKEN`; pushes to
+  `forgejo.coulomb.social/coulomb/forgejo-actions-probe`.
+- `actions/checkout@v4` failed on the host runner — use `git clone` in job steps
+  until checkout action compatibility is confirmed.
 
 ## References