Use REGISTRY_USER secret and build for linux/amd64

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

.gitea/workflows/build.yml

@@ -32,9 +32,9 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - name: Log in to the Gitea container registry
-        run: echo "${{ secrets.REGISTRY_TOKEN }}" | docker login "$REGISTRY" -u "${{ gitea.actor }}" --password-stdin
+        run: echo "${{ secrets.REGISTRY_TOKEN }}" | docker login "$REGISTRY" -u "${{ secrets.REGISTRY_USER }}" --password-stdin
       - name: Build
-        run: docker build -t "$IMAGE:${{ gitea.sha }}" -t "$IMAGE:latest" .
+        run: docker build --platform linux/amd64 -t "$IMAGE:${{ gitea.sha }}" -t "$IMAGE:latest" .
       - name: Smoke test (server refuses to start without accounts, but must import and print usage)
         run: docker run --rm "$IMAGE:latest" python server.py users
       - name: Push