Update Docker images to use ghcr.io (#8774)
Update Docker images to use ghcr.io (#8774)

Companion to https://github.com/dask/dask-docker/pull/207.

We are beginning to publish Docker images to the GitHub Container Registry in addition to Docker Hub, but are switching up documentation to refer to the `ghcr.io` images as we may want to stop publishing to Docker Hub at some point in the future.

For discussion see https://github.com/dask/dask-docker/issues/177
dask/dask,jakirkham/dask,jakirkham/dask,dask/dask