Some alternatives were previously discussed on these threads. Hopefully, they are still relevant till this date.

Tell HN: Heroku alternatives with generous free tiers https://news.ycombinator.com/item?id=32599398

Ask HN: Alternative to Heroku https://news.ycombinator.com/item?id=31258126

Ask HN: Cheaper Heroku alternatives for Rails apps? https://news.ycombinator.com/item?id=30755202