Docker's memory usage of the container is going to be pretty close to the same process running without the namespace. For the hello world they're testing, it's going to be ~ the size of the binary and libc. There's probably another MB or so hiding in Docker metadata, namespace metadata in kernel, etc. that "docker stats" doesn't account for.

depends.

take the hello world example. Correct me if I'm wrong, but if one runs it 1000 times in parallel in a single namspace and it will be using close to 1x memory (i.e. like it was running once, assuming the vast majority of memory would be shared), run it 1000 times in parallel in 1000 docker containers and its memory impact will be closer to 1000x (i.e. no shared memory going on).