image.py: Support KOKKURI_REGISTRY_USER and KOKKURI_REGISTRY_PASSWORD

Ahmon Dancyrequested to merge
main-I71dcbd5916151e0c72b966105fd1643d12da83da into main

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

Related MRs:

Edited by Ahmon Dancy

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