Provide a link to a working Dockerfile example

[Gjacquenot]

Proposed change

Documentation https://mybinder.readthedocs.io/en/latest/tutorials/dockerfile.html is great.

However, providing a link to a working Dockerfile example would be really really great!

Following instructions generally leads to a Binder inaccessible message, as can be seen here:
https://discourse.jupyter.org/t/build-completes-successfully-then-binder-inaccessible/17315

Alternative options

Who would use this feature?

(Optional): Suggest a solution

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