When I am talking with people and reading articles about containers, I hear definition that docker container is isolated one from another, as well as from host machine.
However I wanted to ask, what is exactly isolation level in that case, is it totally isolated from host machine or its OS, or there is some shared resources? If is totally isolated, so how it is so relatively small and lightweight?
A docker container uses the host operating system (kernel) and you can see inside the containers from the host perspective. Usually containers can not see the host processes nor other containers.
Think of container like an isolated application process running, not like a virtual machine.