Docker and Containerization: Basic Concepts and Usage

What is Docker?

Docker is a platform that packages applications and their dependencies into containers. Containers are lightweight, isolated runtime environments.

VM vs Container: Virtual Machines run a full operating system (GBs). Containers share the host OS kernel and take up MBs.

Consistency: Solves the 'works on my machine' problem. Development, test, and production environments use the same container.

Portability: Containers run on any Docker-supported platform (Linux, Windows, Mac, cloud).

Docker Core Concepts

Image: Template for a container. Stored as read-only layers. Built from a Dockerfile.

Container: Running instance of an image. Each container runs as an isolated process.

Dockerfile: Text file that defines how to build an image. Contains instructions like FROM, RUN, COPY, CMD.

Registry: Repository that stores and distributes images. Docker Hub is the most popular public registry.

Volume: Storage used to persist container data. Data is preserved even if the container is deleted.

Writing a Dockerfile

FROM node:18-alpine: Use Node.js 18 (Alpine Linux) as the base image. Alpine is a small-sized base image.

WORKDIR /app: Set the working directory to /app. All subsequent commands run in this directory.

COPY package*.json ./: Copy package.json and package-lock.json. Install dependencies with RUN npm install.

COPY . .: Copy all source code. Create production build with RUN npm run build.

EXPOSE 3000: Specify that the container listens on port 3000. Define the command to run when container starts with CMD ["npm", "start"].

Basic Docker Commands

docker build -t myapp:1.0 .: Build image from Dockerfile. Tag image with name:version using -t.

docker run -p 3000:3000 myapp:1.0: Run container. Map host port to container port with -p.

docker ps: List running containers. docker ps -a: List all containers (including stopped ones).

docker stop <container_id>: Stop container. docker rm <container_id>: Remove container.

docker images: List local images. docker rmi <image_id>: Remove image.

Docker Compose

Docker Compose is a tool for defining and managing multi-container applications. Configured with docker-compose.yml file.

Services: Each service is a container. Define services like web (frontend), api (backend), db (database).

Volumes: Define volumes for data persistence. networks: Create networks for inter-service communication.

docker compose up: Start all services. docker compose down: Stop and remove all services.

docker compose up -d: Run in detached mode (background). docker compose logs -f: Follow logs.

Best Practices

Small images: Use Alpine or distroless base images. Remove build dependencies from final image with multi-stage build.

Layer caching: COPY dependencies (rarely changing) first, code (frequently changing) later. This optimizes build cache.

.dockerignore: Ignore unnecessary files (node_modules, .git, logs). This reduces build context.

Security: Use non-root user instead of root. Create user with RUN adduser --disabled-password appuser.

Health check: Use HEALTHCHECK instruction to perform container health check. Docker automatically restarts unhealthy containers.