ci: route docker.yml to lux-build ARC + drop QEMU arm64 (amd64-only)

The classic [self-hosted, linux, amd64] triple is serviced by fleet arcd hosts
that are not reliably available (v1.2.3 queued 30+ min). Use the in-cluster
lux-build autoscalingrunnerset (same as node + exchange-api). amd64-only — QEMU
arm64 pushed builds to ~2h. Artifact identical to v1.2.3; CI-plumbing only.

.github/workflows/docker.yml

@@ -9,18 +9,19 @@ permissions:
   packages: write
   id-token: write
 
-# Multi-arch on luxfi org's native ARC runner pool.
-# arm64 is provisioned via QEMU until a native arm64 runner is added.
+# amd64-only on luxfi's in-cluster ARC pool (lux-build) — the cluster runs amd64 and
+# QEMU arm64 emulation pushes the build from ~5 min to ~2 h; an arm64 image can return
+# via a native arm64 runner matrix entry, not via QEMU.
 # Semver-only — only v* tags produce {{version}} manifests.
 
 jobs:
   docker:
-    runs-on: [self-hosted, linux, amd64]
+    # In-cluster ARC pool (lux-build autoscalingrunnerset in lux-k8s, org-scoped to
+    # github.com/luxfi, amd64). Replaces the classic [self-hosted, linux, amd64] triple
+    # serviced by fleet arcd hosts that are not reliably available.
+    runs-on: lux-build
     steps:
       - uses: actions/checkout@v4
-      - uses: docker/setup-qemu-action@v3
-        with:
-          platforms: linux/arm64
       - uses: docker/setup-buildx-action@v3
         with:
           driver: docker-container
@@ -41,7 +42,7 @@ jobs:
       - uses: docker/build-push-action@v6
         with:
           context: .
-          platforms: linux/amd64,linux/arm64
+          platforms: linux/amd64
           labels: ${{ steps.meta.outputs.labels }}
           tags: ${{ steps.meta.outputs.tags }}
           cache-from: type=gha,scope=graph