ci: add Gitea Actions workflow to build and push Docker images on tag pushes.

.gitea/workflows/build.yml

@@ -20,7 +20,7 @@ jobs:
         uses: docker/login-action@v3
         with:
           registry: ${{ env.REGISTRY }}
-          username: ${{ var.REGISTRY_USER }}
+          username: ${{ vars.REGISTRY_USER }}
           password: ${{ secrets.REGISTRY_PASSWORD }}
 
       - name: Extract version from tag