Let our Dockerfiles reference a dependency list, and let dependabot keep it updated

I suggest we try to get our installed dependencies in our Dockerfile's be dynamically updated by having a frozen dependency list that gets updated by dependabot submitting PRs that we just press merge on once every week.

By doing that, we get two positive outcomes:

1. Low maintenance effort
2. Version controlled packages in our released images

This issue was created as @TomAugspurger discussed it in https://github.com/dask/dask-gateway/pull/423#pullrequestreview-744011871

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See semi-duplicate #475 that links to examples from jupyterhub/zero-to-jupyterhub-k8s project