What is Testcontainers and how does it relate to the concepts in this book?

Testcontainers is a Java library that programmatically starts real Docker containers (Postgres, Kafka, Redis, etc.) for the duration of an integration test run, then tears them down automatically. It builds directly on Docker Engine's API — the same container lifecycle, networking, and volume concepts from this book — letting integration tests run against real dependency versions instead of mocks or embedded fakes, at the cost of requiring a Docker daemon available in the CI environment.