Passing Heroku config variables to docker build environment

### 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?

Deploying with Docker

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

The only way to have env variables in docker build is by setting all config vars in heroku.yml directly. This is quite problematic because this means we need to push all our build env variables to Github for Github automatic deployments to work.

It would be great if the Heroku config variables can be made available to the build env. So the variables are still defined in the config block of heroku.yml but we can pick these up from the env

```
docker:
web: Dockerfile
config:
ENVIRONMENT_VARIABLE_1 : ${ENVIRONMENT_VARIABLE_1}
```