Intro to virtualization

high level intro into virtualization types: https://virtualizationreview.com/articles/2014/10/14/7-layer-virtualization-model.aspx 

Here is an intro to docker layers: https://windsock.io/explaining-docker-image-ids/  which in turn leads to a common but (as was unknown to me) mechanism called COW or copy-on-write. https://en.wikipedia.org/wiki/Copy-on-write