image-puller assumes that /bin/sh is present

[mattijsjonker]

https://github.com/jupyterhub/zero-to-jupyterhub-k8s/blob/1c9a12cd94eed6842eee985d97126b8ed99e68ce/jupyterhub/templates/image-puller/_helpers-daemonset.tpl#L118-L123

It appears that the image puller assumes that /bin/sh is always available. In my setup this is causing an issue because one of the configured singleuser.extraContainers images is scratch-based, resulting in CrashLookBackoffs on the continuous-image-puller pods:

  image-pull-singleuser-init-and-extra-containers-0:
    Container ID:  containerd://<id>
    Image:         minio/sidekick:v3.0.4
    Image ID:      docker.io/minio/sidekick@sha256:<hash>
    Port:          <none>
    Host Port:     <none>
    Command:
      /bin/sh
      -c
      echo "Pulling complete"
    State:          Waiting
      Reason:       CrashLoopBackOff
    Last State:     Terminated
      Reason:       StartError
      Message:      failed to create containerd task: failed to create shim task: OCI runtime create failed: runc create failed: unable to start container process: exec: "/bin/sh": stat /bin/sh: no such file or directory: unknown
      Exit Code:    128
      Started:      Thu, 01 Jan 1970 00:00:00 +0000
      Finished:     Thu, 17 Aug 2023 10:33:10 +0000
    Ready:          False
    Restart Count:  1
    Environment:  <none>
    Mounts:       <none>

The image in question (https://github.com/minio/sidekick/blob/master/Dockerfile).

I remedied this by creating a custom image that does include /bin/sh, but I wonder if a more elegant solution is possible (e.g., a configurable check).

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[yuvipanda]

Good question. We must have a command here, and one that essentially does nothing (rather than start the default cmd for the image). /bin/sh was chosen as the most common thing to exist here. I'm not sure we can do something else - scratch based containers by definition are unique and usually don't have anything else in them.

Perhaps what we need is to document this requirement?