Decouple docker images

[iameskild]

Fixes | Closes | Resolves - related to #1292

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Changes introduced in this PR:

• Now that the docker images have been transferred to its own repo, we can remove the references here. As a result, a lot of code is being removed.
• With the images transferred, there are few minor changes that were needed in order to properly set the tags for the jupyterlab, jupyterhub and dask-worker images.
  • for testing (kubernetes.yaml), we will pull the latest image tagged from the main branch of nebari-docker-images to ensure that everything from both repos works together. I believe this preserves the spirit of how the tags were set prior to this change.
  • for qhub init, a small function (set_qhub_image_tag) will download a list of the GitHub tags for the nebari-docker-images repo and use the latest (no dev or pre-releases) tag.
    • A similar function has been created to set the qhub_dask_version as well.
  • in both cases, these image tags can be overridden by setting the QHUB_IMAGE_TAG environment variable.
    • A similar env var (QHUB_DASK_VERSION) has been introduced to override the qhub_dask_version.

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[trallard]

Added a few comments, nothing super major @iameskild, thanks for sorting this out

[iameskild]

@trallard I believe this ready for a final review. Once this PR is merged, all future updates to the images will need to be made in the nebari-docker-images repo 👍

[costrouc]

Looks like a good solution moving forward for updating the nebari image tags. Happy to see that we are decoupling the docker images from Nebari!