Docker

What is Docker?

Docker is a containerisation platform that lets you package an application and everything it needs — runtime, libraries, configuration — into a single portable container. Run it on your laptop, on a CI server, or in production and it behaves the same way. That consistency is what made Docker one of the most significant shifts in how software gets deployed.

Containers vs virtual machines

Containers share the host OS kernel rather than virtualising an entire operating system. This makes them much lighter than VMs — they start in seconds, consume less memory, and you can run many more of them on the same hardware. The tradeoff is that containers on the same host share a kernel, so the isolation is less absolute than a full VM. For most application workloads this is fine.

Docker Compose

Compose lets you define multi-container applications in a single YAML file. A typical web application might have a PHP-FPM container, an Nginx container, a MySQL container, and a Redis container — Compose brings them all up together with a single command and handles the networking between them. It's the standard way to run local development environments for multi-service applications.

Images and the registry

Docker images are the templates containers are created from. Docker Hub hosts official images for most common databases, runtimes, and web servers. You build your own images with a Dockerfile that describes what goes on top of a base image. Images are layered and cached, so rebuilds are fast when only the top layers change.

Pricing

Docker is open source and free for most uses. Docker Desktop has licensing restrictions for larger commercial organisations, which prompted some teams to move to alternatives like Colima on macOS. The Docker Engine itself and the CLI remain free.