Docs & release: multi-arch Docker image, README quickstart, RUNNER_BASE_DIRECTORY docs, test.yaml meta fix

[whywaita]

Problems (grouped documentation/release cleanups)

1. Docker image is amd64-only despite arm64 binaries

goreleaser builds linux amd64 and arm64, but Dockerfile pins ENV GOARCH=amd64 and the docker/build-push-action has no platforms, so the published image (ghcr.io/whywaita/myshoes) is amd64-only. Make the image multi-arch to match the released binaries.

2. README lacks quickstart / architecture / config reference

README.md Setup is a single line ("Please see Documents"). There is no quickstart command example, no architecture diagram (myshoes + shoes provider plugin + MySQL + GitHub App), and no environment-variable reference in the README itself.

3. RUNNER_BASE_DIRECTORY is undocumented

Read in pkg/config/init.go (default /tmp) but not mentioned in README or docs/. Every other env var is documented in docs/01_01_for_admin_setup.md.

4. test.yaml references an undefined steps.meta

The docker-build-test job (.github/workflows/test.yaml:47) uses tags: ${{ steps.meta.outputs.tags }}, but there is no id: meta metadata-action step in that job, so tags evaluate to empty (not fatal since push: false, but a misconfiguration).

5. Minor: Dockerfile has a duplicated apk update && apk update.