fix(ci): accelerate multi-platform release builds

[puemos]

Summary

• build AMD64 and ARM64 images in parallel on native GitHub runners
• persist per-architecture BuildKit caches in GHCR and eliminate duplicate main/release builds
• isolate release version, dependency, application, and asset Docker layers

Validation

• full native ARM64 production image build
• version-only rebuild completed in 21.7s with dependency layers cached
• fully warm rebuild completed in 1.8s
• actionlint
• mix format --check-formatted
• git diff --check