image.py: Support KOKKURI_REGISTRY_USER and KOKKURI_REGISTRY_PASSWORD

If KOKKURI_REGISTRY_INTERNAL, KOKKURI_REGISTRY_USER, and
KOKKURI_REGISTRY_PASSWORD are set, set the auth configuration in
~/.docker/config.json accordingly. And make sure this auth info
takes precedence over any JWT auth coming from KOKKURI_JWT.

Likewise for KOKKURI_REGISTRY_CACHE_USER/KOKKURI_REGISTRY_CACHE_PASSWORD.

Bug: T384396

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Related MRs:

• !110 (merged) (This MR)
• !112 (merged)
• !111 (merged)
• main