ci: configure Heroku container stack and environment

- Add step to set Heroku stack to container mode
- Configure container registry login and deployment
- Set up required environment variables for the app

Author: CynthiaWahome

.github/workflows/cd.yml

@@ -20,6 +20,11 @@ jobs:
           HEROKU_API_KEY: ${{ secrets.HEROKU_API_KEY }}
         run: heroku container:login
 
+      - name: Set Heroku stack to container
+        env:
+          HEROKU_API_KEY: ${{ secrets.HEROKU_API_KEY }}
+        run: heroku stack:set container -a github-commit-monitor
+      
       - name: Build and push
         env:
           HEROKU_API_KEY: ${{ secrets.HEROKU_API_KEY }}