How do you use Testcontainers well? H2-vs-real-database, @ServiceConnection, and CI performance.

@SpringBootTest
@Testcontainers
class OrderRepositoryIT {

    @Container @ServiceConnection                  // Boot wires spring.datasource.*
    static PostgreSQLContainer<?> db =
        new PostgreSQLContainer<>("postgres:16-alpine");

    // static = ONE container for the whole class (and JVM, with reuse)
    // Flyway runs the REAL migrations against the REAL engine ✅

    @Autowired OrderRepository repo;

    @Test void upsertUsesPostgresOnConflict() {    // would be untestable on H2
        repo.upsert(order);
        repo.upsert(order.withTotal(Money.of("20")));
        assertThat(repo.findBySku(order.sku())).hasSize(1);
    }
}

// Local dev bonus: @TestConfiguration + @ServiceConnection containers
// power 'bootTestRun' — the app runs locally against containers too.