Shell into a Docker container running on a Heroku dyno. How?

TL;DR Ensure that bash is installed in the image and add this to your Dockerfile:

RUN rm /bin/sh && ln -s /bin/bash /bin/sh

Explanation

Contrary to what the documentation would lead one to believe, Heroku does not, out of the box, support heroku ps:exec into a Docker container already running in a dyno.

Quoting from responses I have received from the Heroku team:

Our ps:exec feature ... works ... by injecting a bash file into dynos, opening an additional port in the background, and allowing you to connect to it.

[T]he default shell used by Docker is /bin/sh, which is not compatible with the Heroku Exec script (it's requires /bin/bash).

There is a workaround you can use though. Put the following in your Dockerfile:

RUN rm /bin/sh && ln -s /bin/bash /bin/sh

This is definitely a gap in our product, and we'll work to make this better.

If bash is installed, run heroku run bash. This will get you into the shell from your command line.

You can also use the GUI and go to "More" -> "Run Console" on your heroku app, and input "bash" to bring it up there.

Edited:In order to run heroku ps:exec on apps with Docker and deployed via the Container Registry you have to enable runtime-heroku-exec.You can do heroku features:enable runtime-heroku-exec to enable it

Here you can see the documentation of exec with the instructions to enable docker support