This project shows how to test a simple Liferay Portlet, with Spring MVC, using PowerMock.
Why not ? :) If you don't like it, you still have all the sources in the git repository.
More seriously, the use of Docker is purely intended to let you test the code without having to download the whole maven world, and Liferay dependencies. It has, for this project, no really other use I can think of.
In a commande line terminal (having docker installed), simply run :
$> docker run -ti --rm afillatre/liferay-springmvc-powermock
-- In docker shell --
docker@docker#> mvn test