fix(ci): rename registry variable to avoid reserved GITEA_ prefix

Gitea rejects variable/secret names starting with GITEA_ or GITHUB_
(reserved for system-injected values), which is why GITEA_REGISTRY
couldn't be created in repo settings.

.gitea/workflows/docker-publish.yml

@@ -20,7 +20,7 @@ jobs:
       - name: Log in to Gitea container registry
         uses: docker/login-action@v3
         with:
-          registry: ${{ vars.GITEA_REGISTRY }}
+          registry: ${{ vars.REGISTRY_HOST }}
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
@@ -30,5 +30,5 @@ jobs:
           context: .
           push: true
           tags: |
-            ${{ vars.GITEA_REGISTRY }}/${{ github.repository }}:latest
-            ${{ vars.GITEA_REGISTRY }}/${{ github.repository }}:${{ github.sha }}
+            ${{ vars.REGISTRY_HOST }}/${{ github.repository }}:latest
+            ${{ vars.REGISTRY_HOST }}/${{ github.repository }}:${{ github.sha }}