Add ability to define environment variables for builder pods.

[ckarwicki]

Hi Binderhub, Jupyterhub Team,

First I would like to say what a great tool Binderhub is. The options it gives, the UI, the possibilities to use different ML tools, languages, packages and it is all open source!!!. Great software.

Proposed change

Not sure if this was already asked but I am trying to set custom environment variable that would be visible in builder pod when repo2docker is running. I am basically trying to set environment variable as PIP_EXTRA_INDEX_URL=https://user:[email protected]/path/api/pypi/python that comes from secret so that pip can install packages from my private repository when building an image based on requirements.txt

Alternative options

PodPresets would have worked but they were removed from Kubernetes 1.20.

Who would use this feature?

Anyone who wants to run notebooks that depend on custom Python packages that needs to be installed from private repository.

(Optional): Suggest a solution

It should be possible to expose env as a Python class traitlet so that it can be set and then append to it in the code. Right now the env is local to submit() function in https://github.com/jupyterhub/binderhub/blob/master/binderhub/build.py#L257 and I am not able to figure how to provide custom environment variable into the builder pod for repo2docker. Any suggestion would be appreciated.

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[manics]

As part of #1318 we are defining an interface for the Builder. This means it'll be much easier to extend and configure. For instance, a new Build.env traitlet could be added and its value set in the standard binderhub_config.py file.