DevOps Lesson 4: Docker

Docker packages your app and ALL its dependencies into a container. “Works on my machine” becomes “works everywhere.” The most important DevOps skill.

Key Concepts

// Image: blueprint (like a class)
// Container: running instance (like an object)
// Dockerfile: recipe for building an image
// Registry: storage for images (Docker Hub, ECR, GCR)

Dockerfile

# Dockerfile
FROM node:20-alpine         # base image

WORKDIR /app                # set working directory

COPY package*.json ./       # copy package files
RUN npm install             # install dependencies

COPY . .                    # copy source code

RUN npm run build           # build app

EXPOSE 3000                 # document the port

CMD ["node", "dist/app.js"] # start command

Docker Commands

# Build image
docker build -t myapp:1.0 .

# Run container
docker run -p 3000:3000 myapp:1.0
docker run -d --name myapp -p 3000:3000 myapp:1.0  # detached

# Manage containers
docker ps              # list running
docker ps -a           # list all (including stopped)
docker logs myapp      # view logs
docker exec -it myapp sh  # shell into running container
docker stop myapp
docker rm myapp

# Images
docker images
docker rmi myapp:1.0
docker pull nginx:latest
docker push myapp:1.0  # push to registry

Multi-stage Build

# Smaller production image
FROM node:20 AS builder
WORKDIR /app
COPY . .
RUN npm install && npm run build

# Final stage: only runtime
FROM node:20-alpine
WORKDIR /app
COPY --from=builder /app/dist ./dist
COPY --from=builder /app/node_modules ./node_modules
CMD ["node", "dist/app.js"]
# Result: 200MB instead of 1GB!