Binder won't launch with `rocker/binder` >= 4.0.0

Hi,

I followed all the instructions from your tutorial. Everything works fine using `rocker/binder` images < 4.0.0, but I always run into an error when trying to use newer versions, 4.0.x:

> 500 : Internal Server Error
> The error was:
> could not start rstudio in time

[My test repo is here](https://github.com/frederic-santos/testholepunch), and as you might observe, the "launch binder" button leads to an error. Reverting to the previous commit, where the image `rocker/binder:3.6.3` is used, is enough to solve the problem.
Am I missing something? Is there anything to do if we want to use the latest rocker images?

---
Also, this is completely unrelated, but you seem to have an extra tab in README.md, at the beggining of the last line below, which triggers an error for me with the `generate_badge()` function.:
```

[](https://mybinder.org/v2/gh/karthik/binder-test/master?urlpath=rstudio)

```

Reproduce the failure using the linked test repository with rocker/binder 4.0.x, then compare it with the working 3.6.3 setup and the reported “could not start rstudio in time” error. Done means the Binder launch succeeds with a newer image; separately check the extra indentation in README.md if that remains in scope.

Written by the indexing model from the issue text.