What is Docker and How It Works — Containers Explained [2026]

Q: What is Docker in simple terms?

Docker is a tool that packages your application and everything it needs to run (code, runtime, libraries, config) into a single unit called a container. You can run this container on any machine that has Docker installed — development laptop, CI server, or cloud VM — and it behaves identically everywhere. It solves the classic "works on my machine" problem.

Q: What is the difference between a Docker image and a container?

A Docker image is a read-only template — like a class definition or a blueprint. It contains the application code, runtime, and all dependencies, but it is not running. A container is a running instance of an image — like an object created from a class. You can run many containers from the same image simultaneously. Images are built once; containers are started, stopped, and destroyed.

Q: How is Docker different from a virtual machine?

A virtual machine (VM) virtualises the entire hardware stack — it includes a full guest operating system (3-10 GB, takes minutes to start). Docker containers share the host OS kernel — only the application and its dependencies are packaged (typically tens of MB, starts in seconds). VMs provide stronger isolation (separate kernel). Containers are lighter and faster but share the host kernel, so a kernel exploit could potentially affect all containers on a host.

Q: What is a Dockerfile?

A Dockerfile is a text file with instructions for building a Docker image. Each instruction creates a layer in the image. Key instructions: FROM (base image), RUN (execute command), COPY (copy files into image), WORKDIR (set working directory), EXPOSE (document port), CMD (default command when container starts), ENV (set environment variable). You run "docker build" to turn a Dockerfile into an image.

Q: What is Docker Compose?

Docker Compose is a tool for defining and running multi-container applications. You describe all your services (web app, database, cache) in a docker-compose.yml file and start everything with one command: "docker compose up". It handles networking between containers, volume mounts, environment variables, and startup order. Compose is ideal for local development environments — it replaces pages of manual docker run commands.

Q: Should I use Docker in production?

Yes — Docker containers are widely used in production. For single-server deployments, Docker Compose or plain Docker works well. For multi-server, high-availability production systems, you would typically use an orchestrator like Kubernetes or Docker Swarm to manage container scheduling, scaling, health checks, and rolling updates. Major platforms like AWS ECS, Google Cloud Run, and Azure Container Apps all run Docker containers natively.

The Problem Docker Solves

Before Docker, deploying an application meant manually installing the right version of Node.js (or Python, Java, PHP) on each server, configuring environment variables, installing libraries, and hoping nothing conflicted with other applications on the same machine. If your app worked locally but not in production, debugging was painful.

Docker packages your app, its runtime, and all dependencies into a container — a portable, isolated environment that runs identically on any machine with Docker installed.

Docker vs Virtual Machines

Feature	Docker Container	Virtual Machine
OS	Shares host OS kernel	Full guest OS (Windows, Linux)
Size	Megabytes (10–500 MB)	Gigabytes (3–20 GB)
Start time	Seconds (sub-second common)	Minutes
Isolation	Process-level (namespace/cgroups)	Full hardware virtualisation
Performance	Near-native (minimal overhead)	5–15% overhead from hypervisor
Portability	Run anywhere Docker is installed	Tied to hypervisor (VMware, HyperV)
Best for	App packaging, microservices, CI/CD	Full OS isolation, different kernels

Core Docker Concepts

Image

A read-only template built from a Dockerfile. Contains the OS base layer, runtime, dependencies, and your application code. Images are immutable — to change an image, you rebuild it. Images are stored in registries (Docker Hub, AWS ECR, GitHub Container Registry).

Container

A running instance of an image. Containers are isolated from each other and from the host, but they share the host OS kernel. You can run thousands of containers on a single host. A container has its own filesystem, network interface, and process space.

Dockerfile

A text file with step-by-step instructions for building an image. Each instruction creates a layer — Docker caches layers, so unchanged layers do not need to be rebuilt, making builds fast.

Registry

A storage and distribution service for Docker images. Docker Hub is the default public registry. You push images to a registry and pull them on any machine.

Writing a Dockerfile

Dockerfile — Node.js app example # Start from official Node.js 20 image FROM node:20-alpine # Set working directory inside container WORKDIR /app # Copy package.json first (separate layer for better caching) COPY package*.json ./ # Install dependencies RUN npm ci --only=production # Copy application code COPY . . # Document which port the app uses EXPOSE 3000 # Command to run when container starts CMD ["node", "server.js"]

Build and run docker build -t my-app:1.0 . # build image docker run -p 3000:3000 my-app:1.0 # run container (map port) docker ps # list running containers docker logs <container-id> # view logs docker stop <container-id> # stop container

Docker Compose for Multi-Container Apps

docker-compose.yml — web app + PostgreSQL + Redis version: '3.8' services: web: build: . ports: - "3000:3000" environment: DATABASE_URL: postgres://user:pass@db:5432/myapp REDIS_URL: redis://cache:6379 depends_on: - db - cache db: image: postgres:16 volumes: - postgres_data:/var/lib/postgresql/data environment: POSTGRES_PASSWORD: pass POSTGRES_USER: user POSTGRES_DB: myapp cache: image: redis:7-alpine volumes: postgres_data:

Start / stop all services docker compose up -d # start all services in background docker compose down # stop and remove containers docker compose logs -f # follow all service logs

Essential Docker Commands

Command	What it does
docker build -t name .	Build image from Dockerfile in current directory
docker run -p 80:80 name	Run container, map host port 80 to container port 80
docker ps	List running containers
docker ps -a	List all containers including stopped
docker images	List local images
docker exec -it <id> sh	Open a shell inside a running container
docker logs -f <id>	Follow container logs
docker rm <id>	Remove a stopped container
docker rmi <image>	Remove an image
docker pull nginx	Pull image from Docker Hub
docker push user/image	Push image to registry

💡 Use .dockerignore

Create a .dockerignore file (similar to .gitignore) to exclude unnecessary files from the build context: node_modules, .git, *.log, .env. This speeds up builds and reduces image size significantly.

⚠️ Never Store Secrets in Docker Images

Do not put API keys or passwords in your Dockerfile or bake them into the image. Use environment variables passed at runtime, Docker secrets, or a secrets manager. Anyone with access to the image can extract its layers and read any secrets baked in.

How We Research and Update This Guide

We test the underlying formula or workflow, compare outputs with reliable references, and revise examples whenever the page content changes.

The workflow or formula is tested directly in the tool and compared against independent reference examples.
Examples are kept practical so readers can verify the result without hidden assumptions.
Pages are revised whenever the interface, calculation flow, or surrounding guidance materially changes.

Frequently Asked Questions — Docker

What is Docker in simple terms?

What is the difference between a Docker image and a container?

How is Docker different from a virtual machine?

What is a Dockerfile?

What is Docker Compose?

Should I use Docker in production?