Add section for docker

[jesseshieh]

Thank you for this website! It's really helpful since it's such a common source of confusion. One thing that's even more confusing is how this works inside a docker container. I think the principles are the same, but tools like top and free are no longer as helpful. With docker becoming more popular, it might make sense to add a section to explain how to check "How do I see how much free ram I really have?" inside a docker container.

[axelabs]

A link to Fabio's Memory inside Linux containers article would be nice.

[TheBritz]

There are concerns for the machine running the docker containers too. Docker containers' processes don't exert pressure on cached memory so the kernel won't release it correctly like it does for regular processes running directly on the machine itself. Containers that are heavy on IO can rapidly fill up the buff/cache and when all the free memory is gone the docker host can go into kernel panic and completely lock up. Seen it countless times due to deploying and working with a large microservice system on a docker host.