ADR-004: Forgejo in-cluster Actions runner on railiance01

Decides long-lived runner Deployment with DinD sidecar; updates
RAIL-HO-WP-0005 runner model decision.

docs/adr/ADR-004-forgejo-in-cluster-actions-runner.md

@@ -0,0 +1,104 @@
+# ADR-004 — Forgejo In-Cluster Actions Runner on railiance01
+
+**Status:** Accepted  
+**Date:** 2026-07-03  
+**Deciders:** Bernd Worsch (operator), custodian agents  
+**Workplans:** `RAIL-HO-WP-0005-T02`, `CUST-WP-0054-T04`
+
+---
+
+## Context
+
+Forgejo production runs on **railiance01 k3s** (`railiance-apps`, S5). An interim
+**host runner** on coulombcore proved Actions scheduling (`coulomb/forgejo-actions-probe`)
+but:
+
+- coulombcore is a legacy machine slated for drain (CUST-WP-0054-T03).
+- Host runners require Docker or Podman on the OS — not installed, not desired on
+  coulombcore long term.
+- Forgejo upstream recommends **not** co-locating runners on the same machine as the
+  forge instance; in-cluster **separate pods** satisfy isolation while staying on the
+  production fleet node.
+- `RAIL-HO-WP-0005-T02` left the runner model undecided among host, in-cluster, and
+  ephemeral options.
+
+Goal: a **coherent Kubernetes-from-the-start** CI substrate — Forgejo app, database,
+ingress, and Actions runner all lifecycle-managed on railiance01.
+
+## Decision
+
+### Runner placement
+
+Deploy **one long-lived Forgejo Actions runner Deployment** in the `forgejo` namespace
+on railiance01:
+
+| Component | Implementation |
+| --- | --- |
+| Runner | `data.forgejo.org/forgejo/runner:6.3.1` |
+| Container runtime for jobs | `docker:dind` sidecar (privileged) |
+| State | PVC `forgejo-runner-data` (`.runner`, `config.yaml`, action cache) |
+| Registration scope | `coulomb` organization |
+| Runner name | `railiance01-build-01` |
+| Deploy surface | `railiance-apps/manifests/forgejo-runner.yaml` |
+| Operator targets | `make forgejo-runner-deploy`, `forgejo-runner-status` |
+
+### Label contract
+
+Preserve Gitea migration compatibility and semantic capability labels:
+
+```text
+self-hosted:host,linux:host,linux_amd64:host,container-build:host,registry-publish:host,railiance01:host,ubuntu-latest:docker://node:20-bookworm,docker:docker://node:20-bookworm
+```
+
+### Security boundaries
+
+- Runner pod receives **no** cluster-admin kubeconfig and **no** OpenBao tokens by default.
+- `registry-publish` jobs use **repo/org-scoped Forgejo secrets** only.
+- DinD sidecar runs **privileged** — accepted for single-node railiance01 with
+  dedicated `forgejo` namespace; revisit when a third node or multi-tenant runners appear.
+- Registration tokens live in Kubernetes Secret `forgejo-runner-registration` (SOPS
+  template committed; live value never in Git).
+
+### Retire interim host runner
+
+Stop and disable `forgejo-runner.service` on coulombcore after in-cluster runner is
+healthy. Do not register new host runners without an explicit ADR amendment.
+
+## Alternatives considered
+
+| Option | Outcome |
+| --- | --- |
+| Host runner + Docker on coulombcore | Rejected — legacy host, contradicts drain plan |
+| Host runner + Podman on haskelseed | Viable fallback; not chosen as primary |
+| Kaniko/Buildah without DinD | Deferred — higher workflow churn during Gitea migration |
+| Multiple ephemeral runner Jobs | Deferred — start with capacity=1 long-lived pod |
+
+## Consequences
+
+**Positive**
+
+- Single-machine production loop: forge + runner on railiance01, workstation not required.
+- Container image CI (`docker build` / `docker push`) works without OS-level Docker.
+- Runner upgrades roll with Git-managed manifests and `kubectl`/Makefile.
+
+**Negative / follow-on**
+
+- Privileged DinD increases blast radius within the node — monitor and restrict namespace RBAC.
+- SOPS-encrypted registration secret still requires operator age key.
+- `cluster-deploy` / `s5-release-check` labels remain **out of scope** until credential paths reviewed.
+
+## Ownership (OAS)
+
+| Concern | Repo | Layer |
+| --- | --- | --- |
+| ADR + umbrella sequencing | `railiance-infra` | S1 |
+| Runner manifests + Makefile | `railiance-apps` | S5 |
+| Label contract + runner evidence docs | `railiance-forge` | S5 forge substrate |
+| Reusable workflow templates | `railiance-enablement` | S4 |
+
+## References
+
+- `railiance-apps/docs/forgejo-on-railiance01.md`
+- `railiance-forge/docs/forgejo-actions-runner-substrate.md`
+- `the-custodian/docs/forgejo-production-decisions.md`
+- [Forgejo runner installation](https://forgejo.org/docs/v11.0/admin/actions/runner-installation/)