ci: use auto-provided GITEA_TOKEN for registry login

Avoids needing to set custom REGISTRY_USERNAME/REGISTRY_TOKEN secrets.
The built-in secrets.GITEA_TOKEN has write:package access for pushing
to the Gitea container registry.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.gitea/workflows/docker-build.yml

@@ -30,8 +30,8 @@ jobs:
         uses: docker/login-action@v3
         with:
           registry: ${{ env.REGISTRY }}
-          username: ${{ secrets.REGISTRY_USERNAME }}
-          password: ${{ secrets.REGISTRY_TOKEN }}
+          username: ${{ gitea.actor }}
+          password: ${{ secrets.GITEA_TOKEN }}
 
       - name: Extract metadata (tags, labels)
         id: meta