Add GitHub Actions CI and local Docker test runner

[ricardclau]

Summary

• replace the old Travis flow with GitHub Actions syntax and integration jobs
• add a local Docker-based test runner for the existing integration suite
• update the SVN integration test to use a temporary local file:// repository fixture instead of GitHub.com SVN access
• document the new CI and local testing workflow

Validation

• ./scripts/test-in-docker.sh syntax
• ./scripts/test-in-docker.sh integration