Summary:
This updates build images to use v5 which I just published, and adds some comments re: updating / using dagster/dagster images.

I think we should do a follow-up to do more involved publishing, where we publish `latest` public images built from master and versioned images for specific Dagster releases, e.g. `dagster/dagster:0.6.0-py37`

Test Plan: buildkite

Reviewers: max, alangenfeld

Reviewed By: max

Differential Revision: https://dagster.phacility.com/D947