Docker for Developers: What You Actually Need to Know

Docker clicked for me the moment I stopped thinking about it as 'containers' and started thinking about it as 'reproducible environments.' The problem Docker solves is real: 'works on my machine' is useless when you're collaborating or deploying. Docker makes environments explicit and repeatable.

The Three Concepts That Actually Matter

Everything in Docker flows from three things: images (blueprints), containers (running instances), and Dockerfiles (instructions to build an image). That's it. Everything else — volumes, networks, Docker Compose — is built on these.

Your First Dockerfile

A Dockerfile is a text file with sequential build instructions. Each instruction creates a layer in the image. Here's a realistic Node.js Dockerfile:

• FROM node:20-alpine — start from an official Node.js image (Alpine = small)
• WORKDIR /app — set working directory inside the container
• COPY package*.json ./ — copy dependency manifests first
• RUN npm ci — install dependencies (this layer is cached when dependencies don't change)
• COPY . . — copy your application code
• EXPOSE 3000 — document which port the app uses
• CMD ["node", "server.js"] — command to run when container starts

Docker Compose for Local Development

For any project with more than one service, use Docker Compose. A simple web app with a database becomes two services in docker-compose.yml: your app and a PostgreSQL container. Define both, run docker-compose up, and they start together on the same network. No manual network configuration, no port conflicts, no 'which postgres am I connecting to.'

The Commands You'll Use 90% of the Time

• docker-compose up -d — start all services in the background
• docker-compose down — stop and remove containers
• docker-compose logs -f [service] — tail logs from a service
• docker ps — list running containers
• docker exec -it [container] sh — open a shell inside a running container
• docker build -t myapp . — build an image from current directory

Common Gotchas to Avoid

Layer caching is Docker's superpower but also its footgun. If you COPY your entire project before installing dependencies, npm install runs every time any file changes. Always copy package.json first, install dependencies, then copy the rest. This way the dependency layer is cached as long as package.json doesn't change.