docs(ci): document workspace bind-mount setup for DooD runners

Add the /srv/gitea-workspace prerequisite step to DEPLOYMENT.md §3.1
and a new "Workspace bind-mount setup" subsection plus failure mode 4
to ci-gitea.md, covering the root cause, one-time host setup, disk
management, and troubleshooting for the bind-mount resolution fix
introduced in ADR-015.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/DEPLOYMENT.md

@@ -193,6 +193,15 @@ curl -fsSL https://tailscale.com/install.sh | sh && tailscale up
 # files to disk during execution (cleaned up unconditionally on completion).
 # A multi-tenant runner would need to switch to stdin-piped env files.
 # (See https://docs.gitea.com/usage/actions/quickstart for the register step.)
+
+# Runner workspace directory — required for DooD bind-mount resolution (ADR-015).
+# act_runner stores job workspaces here so that docker compose bind mounts resolve
+# to real host paths. The path must be identical on the host and inside job containers.
+mkdir -p /srv/gitea-workspace
+# Also add this volume line to the runner service in ~/docker/gitea/compose.yaml:
+#   volumes:
+#     - /srv/gitea-workspace:/srv/gitea-workspace
+# See runner-config.yaml (workdir_parent + valid_volumes + options) and ADR-015.
 ```
 
 ### 3.2 DNS records

docs/infrastructure/ci-gitea.md

@@ -19,6 +19,39 @@ Both containers live in the `gitea_gitea` Docker network on the VPS. The runner
 
 The `gitea-runner` container mounts the host Docker socket (`/var/run/docker.sock`). When a workflow job runs, act_runner spawns a **sibling container** for each job. That job container also gets the Docker socket mounted (via `valid_volumes` in `runner-config.yaml`), enabling `docker compose` calls in workflow steps.
 
+### Workspace bind-mount setup (DooD path resolution)
+
+When a workflow step calls `docker compose up` with relative bind-mount sources (e.g. `./infra/observability/prometheus/prometheus.yml`), Compose resolves them against `$(pwd)` inside the job container and passes the resulting **absolute path** to the host Docker daemon. The host daemon then tries to bind-mount that path from the **host filesystem**.
+
+In the default DooD setup the job container's workspace lives in the act_runner overlay2 layer — the host has no directory at that path, auto-creates an empty one, and the container fails with:
+
+```
+error mounting "…/prometheus/prometheus.yml" to rootfs at "/etc/prometheus/prometheus.yml": not a directory
+```
+
+**Solution (ADR-015):** store job workspaces on a real host path and mount it at the **same absolute path** inside the runner and every job container. `runner-config.yaml` configures this via `workdir_parent`, `valid_volumes`, and `options`.
+
+**One-time host setup** (required on any fresh VPS):
+
+```bash
+mkdir -p /srv/gitea-workspace
+# Then add to the runner service in ~/docker/gitea/compose.yaml:
+#   volumes:
+#     - /srv/gitea-workspace:/srv/gitea-workspace
+# Restart the runner container for the change to take effect.
+```
+
+The path `/srv/gitea-workspace` is the canonical workspace root. It must be identical on the host and inside job containers — if the paths differ, Compose still resolves to the container-internal path, which the host daemon cannot find (the original bug).
+
+**Disk management:** act_runner cleans per-run subdirectories on completion. Orphaned directories from interrupted runs accumulate under `/srv/gitea-workspace` and should be pruned manually if disk space becomes a concern:
+
+```bash
+# List workspace directories older than 7 days
+find /srv/gitea-workspace -mindepth 3 -maxdepth 3 -type d -mtime +7
+```
+
+---
+
 ### Running host-level commands from CI (nsenter pattern)
 
 Job containers are unprivileged and do not share the host's PID/mount/network namespaces. Commands like `systemctl` that target the host daemon are therefore unavailable by default. When a workflow step needs to manage a host service (e.g. `systemctl reload caddy`), it uses the Docker socket to spin up a **privileged sibling container** in the host PID namespace:
@@ -108,6 +141,33 @@ nsenter: failed to execute /bin/systemctl: No such file or directory
 
 The first error means the Docker socket is not mounted into the job container — check `valid_volumes` in `/root/docker/gitea/runner-config.yaml` on the VPS. The second means the Alpine image is running but cannot enter the host mount namespace; verify `--privileged` and `--pid=host` are both present in the workflow step.
 
+**Failure mode 4 — workspace bind-mount not configured (observability stack or any compose-with-file-mounts job)**
+
+Symptom in CI log:
+```
+Error response from daemon: error while creating mount source path "…/prometheus/prometheus.yml": mkdir …: not a directory
+```
+
+Or the service starts but immediately crashes because a config file was mounted as an empty directory.
+
+Cause: `/srv/gitea-workspace` does not exist on the host, or the runner container's `compose.yaml` is missing the `- /srv/gitea-workspace:/srv/gitea-workspace` volume line.
+
+Diagnosis:
+```bash
+ssh root@<vps>
+ls -la /srv/gitea-workspace          # must exist and be a directory
+docker inspect gitea-runner | grep -A5 Mounts   # must show /srv/gitea-workspace
+```
+
+Recovery:
+```bash
+mkdir -p /srv/gitea-workspace
+# Add volume line to runner compose.yaml, then:
+docker compose -f ~/docker/gitea/compose.yaml up -d gitea-runner
+```
+
+See `docs/DEPLOYMENT.md §3.1` and ADR-015 for the full setup rationale.
+
 ---
 
 ## Gitea vs GitHub Actions Differences