Hi Ron,
docker containers are quite different from virtual environments and they are the base for microservices. While in virtual environments you can create programs only based on a specific programming language, containers allow you to create infrastructures with different parts, where each part can be different from the others. This is the reason why they are really loved from developers.
Back your question, containers will always tend to reduce the used resources. If a container ends its job, there’s no reason to get it occuping space or any resources. At any time you can run again the container with the same docker run command. Please remember that if you need to save data you must use persistent volumes as shown in the tutorial (otherwise they will be erased with the container).
An important consideration to do is that containers ARE NOT virtual machines, even if they may appear similar in some cases.

I think that if you try docker containers you will appreciate the flexibility and power of them.

Cheers, Giuseppe