Docker does not use VMs, fix Tooling chapter

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With the caveat that introductory chapters should simplify things by omitting some information, I think it’s too inaccurate to call Docker a VM-related tool. Containers don’t run entire OSes but only a subset of programs. I think it’d be better to call them isolated program environments that contain all of a program’s dependencies.

On a related note, I’m not convinced that the only advantage of containers is to bypass version/packaging problems, there’s also the matter of resource consumption and isolation from other processes. Maybe that too should be clarified.