change user from variables to secrets

.gitea/workflows/docker.yaml

@@ -15,7 +15,7 @@ jobs:
       - name: Login to Gitea Registry
         run: |
           echo "${{ secrets.REGISTRY_TOKEN }}" | docker login gitea.umbra.mom \
-            -u "${{ variables.REGISTRY_USER }}" --password-stdin
+            -u "${{ secrets.REGISTRY_USER }}" --password-stdin
 
       - name: Build image
         run: |