Docker moves fast and breaks things without stabilising them first this post explains the problem fairly well: https://thehftguy.wordpress.com/2016/11/01/docker-in-product....

There have been lots of bugs (some could be considered design failures) allowing processes to escape from a docker container. Fixing those problems hasn't received the attention from the docker community I would expect from a serious OS level virtualisation community. In some cases it boiled down to "yeah just don't do that" or "no problem just put docker inside a VM".