CI: build node images for amd64 as well as arm64 (node-26#28)

amd64 nodes (radio-box) got 'no matching manifest for linux/amd64' on
pull and had to build locally. Every image workflow now builds
linux/amd64,linux/arm64. Release images treats an existing src- tag as
present only if it carries both platforms, so the arm64-only src- tags
pushed before this get rebuilt on the next v1 push.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitea/workflows/build-secondary-sdr.yml

@@ -47,7 +47,7 @@ jobs:
         with:
           context: ./secondary-sdr-container
           file: ./secondary-sdr-container/Dockerfile
-          platforms: linux/arm64
+          platforms: linux/amd64,linux/arm64
           push: true
           tags: |
             git.vpn.cusano.net/${{ vars.DOCKER_ORG }}/${{ steps.meta.outputs.REPO_NAME }}/${{ env.CONTAINER_NAME }}:${{ steps.meta.outputs.VERSION }}