Containers fit very well with CI/CD pipelines. Since they mostly require no further dependencies other than the container runtime itself, they can be easily tested. Worker machines don't have to be provisioned to fulfill the testing needs, so adding more nodes is much easier. What is more, all of them are general-purpose so that they may act both as builders, test runners, and even deployment executors without any prior configuration.
Another great benefit of using containers in CI/CD is the fact that they are isolated from one another. This means multiple copies running on the same machine should not interfere. That is true unless the tests require some resources from the host operating system, such as port forwarding or volume mounting. Therefore it's best to design tests so that such resources are not necessary (or at least they don't clash). Port randomization is a helpful technique to avoid clashes, for example.
