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>

CHANGELOG.md

@@ -19,6 +19,8 @@ The format follows [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ### Changed
 - `test/sample-workflows/docker-login.yml` and `docker-build-push.yml`: now run on the `host` label instead of `ubuntu-latest`. Docker build/push need the runner's bundled Docker CLI and mounted socket, which the `ubuntu-latest` node job container does not provide. Also fixed a doubled image path in the build/push sample (`gitea.repository` already includes the owner).
+- Published `v0.1.0`/`latest` as a **multi-arch** manifest (`linux/amd64` + `linux/arm64`) so it runs on Unraid (amd64) as well as Apple Silicon. The amd64 variant passes the 17/17 smoke test.
+- Unraid docs and compose example: corrected the image path to include the `jason/` owner namespace (`registry.alwisp.com/jason/custom-gitea-runner`), added a required **registry-login** step (the package is private), and recommended the `host` label for Docker build/push jobs (`GITEA_RUNNER_LABELS=…,host:host`).
 
 ### Fixed
 - `docker/runner/Dockerfile`: `COPY` paths for the entrypoint, healthcheck, and smoke-test scripts now resolve from the repo-root build context used by `scripts/build-image.sh` (previously assumed the build context was `docker/runner/`, so the build failed at the `COPY` steps).