fix: write registry credentials directly to docker config.json for BuildKit

.gitea/workflows/docker.yml

@@ -43,11 +43,16 @@ jobs:
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
 
-      # docker/login-action uses OAuth token exchange which Gitea doesn't support.
-      # Use docker login directly for basic auth.
+      # Write credentials directly into config.json so BuildKit can read them
+      # without triggering the OAuth token exchange that Gitea doesn't support.
       - name: Log in to registry
         if: github.event_name != 'pull_request'
-        run: echo "${{ secrets.REGISTRY_PASSWORD }}" | docker login ${{ env.REGISTRY }} -u ${{ secrets.REGISTRY_USER }} --password-stdin
+        run: |
+          mkdir -p ~/.docker
+          printf '{"auths":{"%s":{"auth":"%s"}}}' \
+            "${{ env.REGISTRY }}" \
+            "$(printf '%s:%s' '${{ secrets.REGISTRY_USER }}' '${{ secrets.REGISTRY_PASSWORD }}' | base64 -w0)" \
+            > ~/.docker/config.json
 
       - name: Build and push — standard
         uses: docker/build-push-action@v6