ci: use github.actor + github.token for registry login

github.actor and github.token are the correct Gitea Actions context
variables (gitea.* context doesn't exist in act_runner).

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: ${{ gitea.actor }}
-          password: ${{ secrets.GITEA_TOKEN }}
+          username: ${{ github.actor }}
+          password: ${{ github.token }}
 
       - name: Extract metadata (tags, labels)
         id: meta