I don't think docker is accessible enough right now to kill Heroku. You have to put on your devops hat when you use it. With heroku, you push and it works.
I'm not trying to bash Docker. In fact, I use it quite a bit, and I never use Heroku. I just think that compared to Heroku, it's much more of a low-level tool. With Heroku, you can just paste a few really simple commands and your webapp is now up.