Container

A container is an ordinary process on the server, isolated by the kernel: it sees its own files, its own network and its own process list, and gets a share of CPU and memory. It starts in a fraction of a second because there is no operating system to boot.

How it works

docker run takes an image, adds a thin writable layer on top, creates namespaces (what the process can see) and cgroups (what it may use), then starts the image's command inside. Every container shares the server's kernel: that is why a container is much lighter than a virtual machine, and why a Linux image needs a Linux kernel.

What breaks

A container is not a small server you log into and patch. Anything changed by hand inside it disappears when it is recreated, and nothing outside can reach it until a port is published. Treat containers as disposable: rebuild the image and replace the container.

In a terminal

docker run -d --name web -p 8080:80 nginx:1.27
docker ps
curl http://localhost:8080

See it break, then fix it

Write the Dockerfile or the compose file, type the docker commands, and watch a simulated server run them through reboots, upgrades and traffic. Free to start.

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