ci: remove DockerHub logins

[jameslamb]

Follow-up to #209

Image builds here pull from DockerHub and push to GHCR.

They should not ever need to log in to DockerHub. All requests should be going through our managed proxy cache for DockerHub, which makes requests using its own service account. That use of the proxy happens here for builds:

velox-testing/.github/workflows/build-and-test.yml

Line 141 in 3c09385

buildkitd-config: /etc/buildkit/buildkitd.toml

And automatically for pulls just by being on NVIDIA-hosted runners

velox-testing/.github/workflows/build-and-test.yml

Line 102 in 3c09385

runs-on: linux-${{ matrix.arch }}-cpu16

This PR removes docker login calls to DockerHub

Notes for Reviewers

Benefits of these changes

• removes risk of CI being disrupted by DockerHub credentials expiring, network failures in the auth requests, etc.
• simpler configuration
• less secrets usage = reduced risk of leaking them

How I tested this

I'm unsure how to test whether or not authenticated requests are being made. If CI passes, I recommend merging this and just seeing what happens over a few days of active development.

If you encounter any issues about DockerHub rate limits, conclude I was wrong and revert this.