Support pipeline promotions for container stack apps

### Required Terms

- [X] I agree to follow this project's [Code of Conduct](https://github.com/heroku/roadmap/blob/main/CODE_OF_CONDUCT.md)
- [X] I have read and accept the [Salesforce Program Agreement](https://www.salesforce.com/company/program-agreement/)

### What service(s) is this request for?

Heroku dynos

### Tell us about what you're trying to solve. What challenges are you facing?

[Container deploys](https://devcenter.heroku.com/articles/container-registry-and-runtime) make it easy to build an image inside or outside of Heroku and deploy it to a dyno. However, for apps in a pipeline, there is a [known limitation](https://devcenter.heroku.com/articles/container-registry-and-runtime#known-issues-and-limitations): it is not possible (via UI or API) to promote an app that uses the container stack.

For example, we use this ops flow:
1. A Heroku pipeline containing two stages: `staging` and `prod`
2. Releases are continuously deployed to the `staging` app
3. When ready, a given slug (image) is promoted from `staging -> prod`, deploying a known code checkpoint to the `prod` app

This flow works great until an app is switched to the container stack. Then, step 3 fails because pipeline promotion is not supported.