How to set buildtime environment variables in heroku?

Question

I have an java maven project with docker in heroku in which I want to connect my github repo for automatic deployment.But I have to push some credentials to the app. So I can't use heroku automatic deployment. I decided to pass environment variables (link of the credentials) to the app at the buildstage to get the required credentials. But heroku environment varaibles are limited to runtime only. How to pass environmental variable to the docker container at the build time? or is there any way to push the credentials to the app at the first time then enable automatic deployment without clearing the credentials?