ci-builder: Support running docker inside on macOS

[def-]

See https://materializeinc.slack.com/archives/C07PN7KSB0T/p1765378769080169?thread_ts=1765376951.605999&cid=C07PN7KSB0T

Previously this only supported Linux, which is a blocker for some tasks.

I didn't get this to work fully, currently the /var/run/docker.sock is created as root:root, and thus we are failing permissions:

permission denied while trying to connect to the Docker daemon socket at unix:///var/run/docker.sock: Post "http://%2Fvar%2Frun%2Fdocker.sock/v1.50/images/create?fromImage=docker.io%2Flibrary%2Fredis&tag=latest": dial unix /var/run/docker.sock: connect: permission denied

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.