Add diagnostic step for registry login in Docker build workflow

- Introduce a new step to check registry login credentials, providing feedback on username and token status.
- Implement curl commands to test Basic-Auth and Bearer-Flow for improved debugging during the Docker build process.

.gitea/workflows/docker-build.yml

@@ -28,6 +28,20 @@ jobs:
       - name: Docker Buildx einrichten
         uses: docker/setup-buildx-action@v3
 
+      - name: Registry-Anmeldung prüfen (Diagnose)
+        env:
+          REG_USER: ${{ secrets.REGISTRY_USER || github.repository_owner }}
+          REG_TOKEN: ${{ secrets.REGISTRY_TOKEN }}
+        run: |
+          echo "Registry:      ${REGISTRY}"
+          echo "Username:       '${REG_USER}'  (Laenge: ${#REG_USER})"
+          echo "Token gesetzt:  $([ -n "${REG_TOKEN}" ] && echo ja || echo NEIN)  (Laenge: ${#REG_TOKEN})"
+          echo "--- Test 1: /v2/ mit Basic-Auth ---"
+          curl -s -o /dev/null -w "HTTP %{http_code}\n" -u "${REG_USER}:${REG_TOKEN}" "https://${REGISTRY}/v2/"
+          echo "--- Test 2: Token-Endpoint (Bearer-Flow) ---"
+          curl -s -o /dev/null -w "HTTP %{http_code}\n" -u "${REG_USER}:${REG_TOKEN}" \
+            "https://${REGISTRY}/v2/token?scope=repository:${IMAGE_NAME}:pull,push&service=container_registry" || true
+
       - name: An Gitea Container Registry anmelden
         uses: docker/login-action@v3
         with: