refactor: rename Gitea environment variables to CI registry prefix

update SETUP.md and .env.example

.gitea/workflows/cd.yml

@@ -22,8 +22,8 @@ jobs:
         uses: docker/login-action@v3
         with:
           registry: git.allucanget.biz
-          username: ${{ secrets.GITEA_USERNAME }}
-          password: ${{ secrets.GITEA_TOKEN }}
+          username: ${{ secrets.CI_REGISTRY_USER }}
+          password: ${{ secrets.CI_REGISTRY_TOKEN }}
 
       - name: Pull latest images
         run: |