Add ability to handle imagePullSecrets for private docker registries

[Analect]

@yuvipanda
I was wondering if the kubespawner could be extended to handle a new parameter imagePullSecrets.

My use-case is that I'm using a private registry to contain images. I'm running a cluster on GKE, which enables automatic authentication against the Google Cloud Registry (gcr.io), but not private ones.

As per these docs, there are a couple of work-arounds. Specifically at the section Creating a Secret with a Docker Config, it shows how a secret for the private registry can be created.

Then, in order for an image contained in a kubernetes manifest file to get pulled (in our case, the image used to underpin a notebook server), then additionally the imagePullSecrets parameter needs to be supplied.

Here is an example of what I mean.

In the meantime, if I wanted to hard-code in a secret name, could I just add in that parameter here?

Thanks.

[yuvipanda]

Merged your patch now! Thank you very much!

[Analect]

@yuvipanda ... in case you didn't notice, just a heads-up .. the patch that @shraykay added here ended up not getting merged, since I think it was missing the new parameter in test/test_objects.py and failed in the build pipeline.