The workspace-full takes a long time to docker pull in a first run of gp rebuild

[loujaybee]

When running gp rebuild for the first time, the docker pull takes quite a long time, as it's typically using workspace-full as the default image, which takes a long time to pull. We should find ways to encourage users to build smaller, more slimline workspace images, or re-using our existing workspace-images.

[stale]

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