Comment by sammnaser

Different Linux distributions ship with various package managers / other userspace tooling. But they all run on top of the Linux kernel. I can see how having a base image called "Ubuntu 18.04" might make it seem like there's an entire OS running in the container but that's not the case. Check out this Ubuntu base image Dockerfile https://github.com/tianon/docker-brew-ubuntu-core/blob/bcdd3..., you can see that it's throwing a tarball for the root filesystem into the container (and some Docker-specific stuff), nothing crazy.