What Is Docker? A Beginner’s Guide to Containers, Images, and Compose

What is Docker?

Docker is an open‑source container engine that lets developers package applications and their dependencies into portable images, which can be run on any Linux or Windows host. Containers are isolated using sandbox mechanisms.

Service Model

Docker follows a client‑server (C/S) architecture. The Docker client communicates with the Docker daemon (server) to manage images and containers.

Communication Method

Docker uses a Unix socket, e.g., unix:///var/run/docker.sock, as the default connection endpoint.

Core Components

Docker Client

Docker Daemon (service)

Docker Image

Docker Container

Docker Hub (Repository)

A repository is a centralized storage for images. Docker Hub is the public repository maintained by Docker, from which most users pull images.

What Is an Image?

An image is a read‑only template for the Docker engine, containing a filesystem. It can represent a full OS environment or include additional applications. Images are versioned and built incrementally.

Building an Image

Images are built with a Dockerfile, which describes the steps to assemble the filesystem, install dependencies, and copy source code. The Docker daemon reads the Dockerfile and executes docker build . to produce a custom image. If an image is pulled from a registry, a Dockerfile is not required.

Running Containers

Containers are started from images using commands such as docker run for a single container or docker‑compose up for multiple containers. The run command includes options for port mapping, volume mounting, environment variables, etc.

Orchestrating Multiple Containers

Docker‑Compose defines multi‑container applications in a docker‑compose.yml file, specifying which images to use, their configuration, volumes, ports, and scaling options. With a single docker‑compose up command, all services are launched together, simplifying management compared to issuing separate docker run commands.

Dockerfile vs docker‑compose.yml

Dockerfile

records the build steps for a single image. docker‑compose.yml describes how multiple images are combined and run as services.