fix: use config.json auth for Gitea registry login

docker login --password-stdin fails on act runner (non-TTY).
Use base64 auth in ~/.docker/config.json instead, matching
the kuberay-images pattern.

.gitea/workflows/build-push.yaml

@@ -138,7 +138,18 @@ jobs:
 
       - name: Login to Gitea Registry
         run: |
-          echo "${{ secrets.REGISTRY_TOKEN }}" | docker login ${{ env.REGISTRY_HOST }} -u ${{ secrets.REGISTRY_USER }} --password-stdin
+          AUTH=$(echo -n "${{ secrets.REGISTRY_USER }}:${{ secrets.REGISTRY_TOKEN }}" | base64 -w0)
+          mkdir -p ~/.docker
+          cat > ~/.docker/config.json << EOF
+          {
+            "auths": {
+              "${{ env.REGISTRY_HOST }}": {
+                "auth": "$AUTH"
+              }
+            }
+          }
+          EOF
+          echo "Auth configured for ${{ env.REGISTRY_HOST }}"
 
       - name: Extract metadata
         id: meta