Docker rate limiting causes CI to fail

[sharnoff]

When there's a lot of commits in a short period, we sometimes get rate limited by docker because we have unauthenticated image pulls in CI.

This causes CI to fail - and retrying doesn't help, because the issue is we're exceeding the rate limit 😞

It would be nice if this didn't happen, so CI didn't fail :)

Tasks

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1. Implement docker auth for vm-builder #814
   a/docker t/feature +2
   
2. Docker rate limiting causes e2e-tests to fail #975
   a/ci +1