Start real dependencies inside the test run and throw them away afterward. Testcontainers launches disposable Docker containers for databases, brokers and other services from your test code, so each run or worker gets its own isolated instance, tests can run in parallel, and behavior matches production more closely than mocks or in-memory substitutes. It also gives developers the same integration tests locally and in CI without a shared staging database that drifts or gets polluted. Compared with Docker Compose, it avoids port conflicts and manual lifecycle handling. Trade-offs are the need for Docker in CI and slower startup than pure unit tests, so keep it for integration tests.

Sources: Testcontainers, Docker blog, doubleSlash, DEV Community