Buildkit runs in isolated network and can't access mirrors.
Legacy builder uses host daemon directly with registry-mirrors.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Buildkit ignores daemon.json registry-mirrors, so specify
mirror directly in FROM statements.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>