Docker containers don't change the functionality of a containerized application compared to a non-containerized one, but they do introduce new challenges such as different operating system requirements, security concerns, and testing complexities for microservices. Containers require separate branches for each operating system, and security certifications need to be considered when building containers. Testing for microservices becomes more challenging due to the need to test individual APIs, but tools like Postman and Newman can help automate API tests. Overall, containerization brings better reuse and change tracking, but also presents some bumps in the road for quality testing engineers.