mzcompose: Default to using GHCR mirror

[def-]

It has so far worked well in CI and for me locally

One concern is that people could be logged into DockerHub, but not GHCR, and would thus have a lower quota as a consequence of this PR. Set MZ_GHCR=1 if you want to try it without this PR, and MZ_GHCR=0 to go back to DockerHub after this PR is merged.

https://materializeinc.slack.com/archives/CM7ATT65S/p1762165581088089?thread_ts=1761297962.236689&cid=CM7ATT65S

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.