Docker images are layered. The bottom layer is the operating system, which can be a full OS like Windows Server Core, or a minimal OS like Microsoft Nano Server. On top of this are layers for each change you make to the base OS when you build an image, by installing software, copying files, and running commands. Logically, Docker treats the image as a single unit, but physically each layer is stored as a separate file in Docker's cache, so images with a lot of common features can share layers from the cache.

Images are built using a text file with the Dockerfile language – specifying the base OS image to start with and all the steps to add on top. The language is very simple, and there are only a few commands you need to master in order to build production-grade images. I'll start by looking at the basic PowerShell image I've been using so far in this chapter.