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Docker Cheat Sheet

Containerization Made Simple

1. Core Concepts

  • Image: Immutable template with app code, dependencies, and config.
  • Container: Isolated runtime instance of an image.
  • Dockerfile: Blueprint to automate image builds.
  • Registry: Hub for storing/sharing images (e.g., Docker Hub, ECR).
  • Volume: Persistent storage for containers.
  • Network: Communication channel between containers/host.

2. Installation

Linux

curl -fsSL https://get.docker.com | sh  
sudo usermod -aG docker $(whoami) && newgrp docker

macOS/Windows

Verify

docker version          # Check client/server versions  
docker run hello-world  # Run a test container

3. Image Management

Command Description
docker images List local images
docker pull <image>:<tag> Download image from registry
docker rmi <image> Remove image
docker build -t <name>:<tag> . Build image from Dockerfile
docker image prune -a Delete unused images

Example:

docker pull nginx:alpine  # Lightweight Nginx image  
docker build -t myapp:1.0 .  # Build from current directory's Dockerfile

Note:

  • Use :<tag> (e.g., :latest, :alpine) to specify image versions.
  • docker image prune -a removes all unused images, not just dangling ones.

4. Container Lifecycle

Command Description
docker run -d --name <name> <image> Run container in background
docker start/stop/restart <name> Manage container state
docker rm -f <name> Force remove running container
docker exec -it <name> sh Open interactive shell
docker logs -f <name> Tail container logs
docker ps -a List all containers (including stopped)

Common Flags:

  • -p 8080:80: Map host port 8080 → container port 80.
  • -v /host/path:/container/path: Bind mount volume.
  • -e VAR=value: Set environment variable.
  • --network mynet: Attach to custom network.

Example:

docker run -d --name webserver -p 8080:80 -v ./html:/usr/share/nginx/html nginx:alpine

Explanation:

  • -d: Run in detached mode (background).
  • -v ./html:/usr/share/nginx/html: Mount the local html folder into the container’s Nginx directory.

5. Dockerfile Essentials

Sample Dockerfile

FROM python:3.9-slim                 # Base image  
WORKDIR /app                         # Set working directory  
COPY requirements.txt .              # Copy dependencies file  
RUN pip install --no-cache-dir -r requirements.txt  # Install deps  
COPY . .                             # Copy app code  
EXPOSE 5000                          # Declare port  
CMD ["gunicorn", "--bind", "0.0.0.0:5000", "app:app"]  # Runtime command

Multi-Stage Build

# Build stage (heavy, includes build tools)  
FROM node:18 as builder  
WORKDIR /app  
COPY package*.json ./  
RUN npm install  
COPY . .  
RUN npm run build  # Compile app  

# Production stage (lightweight, only necessary files)  
FROM nginx:alpine  
COPY --from=builder /app/dist /usr/share/nginx/html  # Copy built files

Build & Run:

docker build -t myapp:1.0 .  
docker run -d -p 80:80 myapp:1.0

Note:

  • Multi-stage builds reduce image size by discarding build tools in the final image.
  • --from=builder copies files from the earlier build stage.

6. Networking

Command Description
docker network ls List networks
docker network create mynet Create custom network
docker network inspect mynet Show network details

Example:

docker network create app_network  
docker run -d --network app_network --name frontend nginx  
docker run -d --network app_network --name backend api:latest

Explanation:

  • Containers on the same network can communicate using their service names (e.g., frontend can ping backend).

7. Volumes & Storage

Command Description
docker volume create myvol Create named volume
docker volume ls List volumes
docker run -v myvol:/data ... Mount volume

Example:

docker run -d --name db -v pgdata:/var/lib/postgresql/data postgres:15

Note:

  • Volumes persist data even if the container is deleted.
  • Use -v pgdata:/var/lib/postgresql/data to store PostgreSQL data permanently.

8. Docker Compose

Sample docker-compose.yml

version: '3.8'  
services:  
  web:  
    image: nginx:alpine  
    ports:  
      - "80:80"  
    volumes:  
      - ./html:/usr/share/nginx/html  # Bind mount for static files  
  db:  
    image: postgres:15  
    environment:  
      POSTGRES_PASSWORD: secret  # Set DB password  
    volumes:  
      - pgdata:/var/lib/postgresql/data  # Persistent volume  

volumes:  
  pgdata:  # Define named volume
Command Description
docker compose up -d Start services in background
docker compose down Stop and remove containers/volumes
docker compose logs -f Follow service logs

Explanation:

  • docker compose up -d reads the docker-compose.yml file and starts all defined services.
  • Named volumes (e.g., pgdata) are managed automatically.

9. Advanced Techniques

Resource Limits

docker run -d --name app --memory=512m --cpus=1.5 myapp:1.0

Note:

  • Restrict memory (--memory) and CPU (--cpus) to prevent a container from hogging resources.

Health Checks

HEALTHCHECK --interval=30s --timeout=3s \  
  CMD curl -f http://localhost:5000/health || exit 1

Note:

  • Docker periodically runs this check to verify the app is healthy.
  • Use docker inspect <container> to see health status.

Security Best Practices

  1. Avoid root user in containers: 
    FROM node:18  
USER node  # Run as non-root "node" user
  2. Scan for vulnerabilities: 
    docker scan myapp:1.0  # Uses Snyk to find CVEs

10. Maintenance & Optimization

Command Description
docker system prune -a --volumes Remove unused images/containers/volumes
docker stats Live resource usage monitor
docker history <image> Inspect image layers

Tips:

  • Use .dockerignore to exclude unnecessary files (e.g., node_modules, .git).
  • Tag images semantically: myapp:1.0-prod.

11. Troubleshooting

Issue Solution
Port conflicts Check running containers: docker ps
Permission denied Use -v $(pwd):/data:ro for read-only mounts
Container won’t start Inspect logs: docker logs <name>
Image too large Use multi-stage builds and Alpine-based images

12. References