Make v0.1.0 Unraid-ready: multi-arch image, correct image path, registry-login docs

- Publish v0.1.0/latest as multi-arch (linux/amd64+arm64); amd64 passes 17/17 smoke
- publish-image.sh: add PLATFORMS multi-arch buildx mode (reproducible amd64 build)
- compose + unraid notes: fix image path to include jason/ namespace
- unraid notes: document required docker login (private package) and host label
  for docker build/push jobs

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker/compose/docker-compose.example.yml

@@ -7,7 +7,7 @@
 
 services:
   gitea-runner:
-    image: registry.alwisp.com/custom-gitea-runner:v0.1.0  # pin your version here
+    image: registry.alwisp.com/jason/custom-gitea-runner:v0.1.0  # pin your version here (Gitea CR requires the owner namespace)
     container_name: gitea-runner
     restart: unless-stopped
 
@@ -18,7 +18,9 @@ services:
       GITEA_RUNNER_TOKEN: ${GITEA_RUNNER_TOKEN}      # set in .env file
 
       # ── Optional ──────────────────────────────────────────────────────────
-      GITEA_RUNNER_LABELS: ubuntu-latest:docker://node:20-bullseye
+      # `host` runs steps in the runner container (Docker CLI + socket available),
+      # needed for docker build/push. `ubuntu-latest` runs in a clean node container.
+      GITEA_RUNNER_LABELS: ubuntu-latest:docker://node:20-bullseye,host:host
       RUNNER_CONFIG_PATH: /config/runner.yaml
       RUNNER_WORKDIR: /work
       TZ: America/Chicago