Improve macOS local development support for docker tests

[shamardy]

Context

When running docker tests locally on macOS, there are a few issues:

1. Zcash params path mismatch: The fetch script puts params in ~/Library/Application Support/ZcashParams but docker-compose expects ~/.zcash-params. Users need to symlink or set ZCASH_PARAMS_PATH env var.

2. ARM/M1 Cosmos images: Cosmos images are amd64-only and require Rosetta emulation. See discussion: refactor(ci): split docker tests into feature-gated parallel suites #2707 (comment)

Proposed improvements

• Document the zcash-params workaround for macOS users
• Consider building native arm64 Cosmos images
• Add macOS-specific setup instructions to DOCKER_TESTS.md

Related

PR #2707 - Docker tests infrastructure