ci(docker): publish multi-arch image (linux/amd64 + linux/arm64) (#503)

* ci(docker): publish multi-arch image (linux/amd64 + linux/arm64)

Apple Silicon users hit "no matching manifest for linux/arm64/v8"
when running `docker compose up -d` because the publish workflow
defaulted to the GHA runner's native arch (amd64). Add a QEMU setup
step and tell buildx to emit both linux/amd64 and linux/arm64
manifests under the same tag.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* docs(self-hosting): drop stale migration file count

The doc claimed "63 ordered SQL files" — actual count is ~210 and
growing every release. Removing the specific number so it stops drifting.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/docker-publish.yml

@@ -34,6 +34,9 @@ jobs:
             type=raw,value=latest
             type=sha,prefix=
 
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
 
@@ -41,6 +44,7 @@ jobs:
         uses: docker/build-push-action@v6
         with:
           context: .
+          platforms: linux/amd64,linux/arm64
           push: true
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}