For a dummy programming analogy, you can think of Docker has a abstract ImageFactory which holds ImageFactories they come from store.
Then once you want to create an app out of that ImageFactory, you will have a new container, and you can modify it as you want. DotNetImageFactory will be immutable, because it acts as a abstract factory class, where it only delivers instances you desire.
IContainer newDotNetApp = ImageFactory.DotNetImageFactory.CreateNew(appOptions);
newDotNetApp.ChangeDescription("I am making changes on this instance");
newDotNetApp.Run();