How to use Docker as an isolated environment for running automated tests and CI pipelines?

Using containers to execute automated tests guarantees absolute environment parity between the developer's machine and the continuous integration server. This completely eliminates the classic problem where tests pass successfully locally but fail on a remote server due to differences in system library versions.

The process of organizing testing is usually built on a special build configuration file that describes the creation of a temporary test container. The project's source code is copied into this container, dependencies are installed, and the test suite is executed with a corresponding exit code.

To test applications that depend on databases or message queues, multi-service orchestration is used. Automation tools allow spinning up a clean database in an isolated network, waiting for it to be ready, running the application's integration tests, and cleanly removing all temporary resources upon completion.

An important optimization in this approach is the proper use of dependency layer caching. If the file describing the project dependencies has not changed, the installation phase is skipped, which reduces the execution time of the testing pipeline by several times and accelerates code delivery to production.