docker

One command hides four layers of software and a set of standards that made them interchangeable. This lesson takes the stack apart, then asks the question the whole path has been building toward: given a shared kernel, how much is container isolation actually worth, and what has to be added before it is a security boundary.

Isolation is the point, and a container that can reach nothing and keep nothing is useless. This lesson covers how a network namespace is wired to the outside, why published ports and container-to-container traffic work completely differently, and where data has to live given that the writable layer disappears.

The kernel gives isolation for files already on the machine. The image format is what turned a directory tree into something you can build once, name by digest and run anywhere. This lesson covers layers and the union filesystem that stacks them, why the build cache behaves as it does, and the content addressing that makes it all work.

There is no container object in the Linux kernel, no container system call, and no container data structure. A container is an ordinary process started with several isolation features switched on at once. This lesson establishes what those features are, what the word actually names, and why the distinction changes how you reason about them.