fix: enable Nix experimental features in docker-build-nix

Add Nix config step to enable nix-command and flakes features
before building. Also configures Attic substituters for faster builds.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

docker-build-nix/action.yaml

@@ -42,6 +42,16 @@ inputs:
 runs:
   using: composite
   steps:
+    - name: Configure Nix
+      shell: bash
+      run: |
+        mkdir -p ~/.config/nix
+        cat > ~/.config/nix/nix.conf <<EOF
+        experimental-features = nix-command flakes
+        extra-substituters = https://cache.nixos.org/ ${{ inputs.attic-endpoint }}/ci ${{ inputs.attic-endpoint }}/toph
+        extra-trusted-public-keys = cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY= ci:db8ZBxd5cjqoGzOYThRQcxj4XnaqHJZBZw1phCQOiz8= toph:E/oP7KyljH/yprI5LArxNPpSlQCdo29sMOkh3jm53Yg=
+        EOF
+
     - name: Build OCI image with Nix
       shell: bash
       run: |