Support ...image.pull[Policy|Secrets] consistently and hook into JupyterHub's chart wide configuration for imagePullSecrets

[emrahkaya]

Fixing the unused image.pullSecrets value in the binderhub Deployment.

Update by Erik

Thanks to jupyterhub/zero-to-jupyterhub-k8s#2546, we can hook into z2jh's chart wide configuration of imagePullSecrets. With this PR, we do that for all our pods where we specify an image.

I also added support for image.pullPolicy which wasn't supported in practice even though the schema supported it.

[emrahkaya]

Our tests here doesn't cover this case, but I've tested this my system, using a private registry.

In my tests, I have put this step in the test.yml:

      - name: Create an image pull secret
        run: |
          kubectl create secret docker-registry acr-secret \
            --namespace default \
            --docker-server="${{ secrets.DOCKER_REGISTRY }}" \
            --docker-username="${{ secrets.DOCKER_USERNAME }}" \
            --docker-password="${{ secrets.DOCKER_PASSWORD }}"

and modified testing/k8s-binder-k8s-hub/binderhub-chart-config.yaml file, adding

image:
  pullSecrets:
    - acr-secret

there.

Such a test might not apply here, but I'm putting this information for anyone interested using/testing it (or a proof for my tests :) ).

[welcome]

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