Update Docker build workflow to use secrets for registry authentication

- Change username and password fields in the Docker login action to utilize secrets for enhanced security.
- Ensure compatibility with different registry user configurations by allowing fallback to the repository owner.

.gitea/workflows/docker-build.yml

@@ -32,8 +32,8 @@ jobs:
         uses: docker/login-action@v3
         with:
           registry: ${{ env.REGISTRY }}
-          username: ${{ github.actor }}
-          password: ${{ secrets.GITHUB_TOKEN }}
+          username: ${{ secrets.REGISTRY_USER || github.repository_owner }}
+          password: ${{ secrets.REGISTRY_TOKEN }}
 
       - name: Metadaten (Tags & Labels) bestimmen
         id: meta