What a container actually is — a process the kernel has fenced off, not a small machine. Why containers beat the "works on my machine" problem, how they differ from virtual machines, the namespaces and cgroups that make them work, the image-versus-container distinction the whole book turns on, the architecture under the docker command, and your first running container.
6 topics
Docker is a small idea wrapped in a lot of vocabulary. The idea: a container is an ordinary process the kernel has been told to isolate, and an image is the read-only artifact you build it from. Everything else in this course — Dockerfiles, networking, volumes, Compose — is detail in service of that one distinction.
This chapter builds the mental model before you run anything in anger. What containers solve and why hand-installed environments rot, how a container differs from a virtual machine, the two kernel features that make isolation possible, the difference between an image and a container, the client-daemon-runtime chain under the docker command, and the first container you run on your own host. The rest of the course assumes these six pages.