Docker

Docker is an application packager that enables you to run applications in isolated containers with all their dependencies.

Quick Reference

Common Docker commands for daily use.

Command	What it does
`docker run -d -p 8080:80 --name c nginx`	Create and start container
`docker ps` / `docker ps -a`	List running / all containers
`docker stop / start / restart c`	Stop / start / restart container
`docker rm -f c`	Force remove container
`docker logs -f c`	Follow logs in real-time
`docker exec -it c bash`	Open terminal in container
`docker images`	List local images
`docker pull nginx:alpine`	Download image from Docker Hub
`docker build -t app:1.0 .`	Build image from Dockerfile
`docker rmi nginx`	Remove image
`docker volume ls / rm / prune`	List / remove / clean volumes
`docker network ls / create / rm`	Manage networks
`docker compose up -d`	Start all services
`docker compose down`	Stop all services
`docker compose logs -f app`	Service logs
`docker compose exec db psql ...`	Execute command in service
`docker system df`	Space used by Docker
`docker system prune -a`	General cleanup

Core Concepts

Application packager that bundles code and dependencies into portable containers.

Image

Template for a container. Defines structure once and use multiple times easily. Stored locally or on hub.docker.com.

Container

Running instance of an image. The actual process/execution on the system. Multiple containers can run from the same image. Data is not stored (lost when container stops).

Volume

Where data persists in Docker. Stores data outside the container so it survives restarts.

Dockerfile

A file containing instructions for building an image. Replaces manual command-line work.

Docker Run

Downloads image, creates and starts a container.

Docker Run Flags

Flag	What it does
`-d` (—detach)	Runs in background. Without this, terminal is locked to container.
`-p HOST:CONTAINER`	Maps ports. Ex: `-p 8080:80` — port 8080 on your PC routes to 80 in container.
`--name NOME`	Gives container a name. Makes it easier to reference later.
`-e VAR=VALOR`	Sets environment variable inside container. Ex: `-e POSTGRES_USER=myuser -e POSTGRES_PASSWORD=mypass123`
`-v ORIGEM:DESTINO`	Mounts a volume — connects host folder with container folder. Ex: `-v postgres-data:/var/lib/postgresql/data`
`--rm`	Removes container automatically when it stops.
`-it`	Interactive mode with terminal. Use to enter the container.
`--network REDE`	Connects container to a specific network.
`--restart always`	Restarts container automatically if it stops or host reboots.

At the end of the command, specify the image to use (e.g., nginx, postgres:15, node:20-alpine).

docker run -d -p 8080:80 --name web nginx

Managing Containers

Daily Commands

List containers

# Running containers
docker ps

# All containers (including stopped)
docker ps -a

# Custom format (more readable)
docker ps --format 'table {{.Names}}\t{{.Status}}\t{{.Ports}}'

Start / Stop

# Stop gracefully (sends SIGTERM, waits 10s, then SIGKILL)
docker stop meu-nginx

# Stop immediately (SIGKILL direct — use only if necessary)
docker kill meu-nginx

# Start stopped container
docker start meu-nginx

# Restart
docker restart meu-nginx

Remove

# Remove stopped container
docker rm meu-nginx

# Force remove running container
docker rm -f meu-nginx

# Remove all stopped containers
docker container prune

Logs

# View all logs
docker logs meu-nginx

# Follow logs in real-time (like tail -f)
docker logs -f meu-nginx

# View only last 50 lines
docker logs --tail 50 meu-nginx

# Logs with timestamp
docker logs -t meu-nginx

Enter container

# Open bash terminal in container
docker exec -it meu-nginx bash

# If image doesn't have bash, try sh
docker exec -it meu-nginx sh

# Execute single command without entering
docker exec meu-nginx ls /var/www/html

Inspect

# Detailed container information (JSON)
docker inspect meu-nginx

# View CPU, memory and network usage in real-time
docker stats

# Stats for specific container
docker stats meu-nginx

Managing Images

List and pull

# List locally downloaded images
docker images

# Download image without creating container
docker pull postgres:15
docker pull node:20-alpine

# Remove an image
docker rmi nginx

# Remove unused images (dangling)
docker image prune

# Remove ALL unused images
docker image prune -a

# Search for image on Docker Hub via terminal
docker search postgres

Volumes

Volume Types

Type	When to use
Named Volume (recommended)	Managed by Docker. Located in /var/lib/docker/volumes/. Best for databases.
Bind Mount	Connects specific host folder to container. Ideal for development (see changes live).
tmpfs Mount	Storage in RAM memory. Temporary, very fast. For sensitive data in memory.

Named Volumes in Practice

Create and use volumes

# Create a volume manually
docker volume create dados-postgres

# List volumes
docker volume ls

# Inspect a volume (see physical location, etc)
docker volume inspect dados-postgres

# Use the volume in container
docker run -d \
  --name postgres \
  -v dados-postgres:/var/lib/postgresql/data \
  -e POSTGRES_PASSWORD=senha \
  postgres:15

# If you remove and recreate container, data persists!
docker rm -f postgres
docker run -d \
  --name postgres \
  -v dados-postgres:/var/lib/postgresql/data \
  -e POSTGRES_PASSWORD=senha \
  postgres:15
# Data is still there!

Removing a volume deletes all data permanently!

Remove volumes

# Remove a volume (CAREFUL: deletes data!)
docker volume rm dados-postgres

# Remove unused volumes
docker volume prune

Bind Mounts — Local Development

Bind mounts are essential for development. They mirror your local folder inside the container — any code changes are reflected immediately without rebuilding the image.

Development with bind mount

# Mount current folder inside container
docker run -d \
  --name app-dev \
  -p 3000:3000 \
  -v $(pwd):/app \
  -v /app/node_modules \
  node:20-alpine \
  sh -c 'npm install && npm run dev'

# What happened:
# -v $(pwd):/app          = current host folder mapped to /app in container
# -v /app/node_modules    = anonymous volume for node_modules (won't overwrite!)

# On Windows PowerShell, use ${PWD} instead of $(pwd)
docker run -v ${PWD}:/app ...

Dockerfile

Create your own custom images.

Dockerfile Instructions

Instruction	What it does
`FROM imagem:tag`	Defines base image. Always the first instruction.
`WORKDIR /caminho`	Sets working directory inside container.
`COPY origem destino`	Copies files from host to container.
`ADD origem destino`	Like COPY, but supports URLs and automatically extracts tar files.
`RUN comando`	Executes command during image BUILD (install packages, etc).
`CMD ["cmd", "arg"]`	Default command when starting container. Can be overridden.
`ENTRYPOINT ["cmd"]`	Fixed entry point of container. CMD becomes its arguments.
`ENV VAR=VALOR`	Defines environment variable available at runtime.
`ARG VAR=VALOR`	Variable available ONLY during build time.
`EXPOSE porta`	Documents which port container will use (doesn’t open automatically).
`VOLUME /caminho`	Declares a volume mount point.
`USER usuario`	Defines user that runs subsequent commands (security).

Example Dockerfile

Multi-stage Node.js Dockerfile

# ─────────────────────────────────────────────────────────────
# STAGE 1: Install dependencies
# ─────────────────────────────────────────────────────────────
FROM node:20-alpine AS dependencies

# Install native dependencies if necessary
# (bcrypt, sharp and others need build tools)
RUN apk add --no-cache python3 make g++

WORKDIR /app

# Copy ONLY dependency files first
# This leverages Docker cache:
# If package.json didn't change, this layer is reused
COPY package.json package-lock.json ./

# Install production dependencies
RUN npm ci --only=production


# ─────────────────────────────────────────────────────────────
# STAGE 2: Final image (smaller and more secure)
# ─────────────────────────────────────────────────────────────
FROM node:20-alpine

# Create non-root user to run application (security!)
RUN addgroup -S appgroup && adduser -S appuser -G appgroup

WORKDIR /app

# Copy node_modules from previous stage
COPY --from=dependencies /app/node_modules ./node_modules

# Copy application code
COPY . .

# Change file owner to created user
RUN chown -R appuser:appgroup /app

# Switch to non-root user
USER appuser

# Document port (good practice, not mandatory)
EXPOSE 3000

# Environment variables with default values
ENV NODE_ENV=production
ENV PORT=3000

# Check if application is healthy
HEALTHCHECK --interval=30s --timeout=5s --start-period=15s --retries=3 \
  CMD node -e "require('http').get('http://localhost:3000/health', (r) => process.exit(r.statusCode === 200 ? 0 : 1))"

# Start application
CMD ["node", "src/server.js"]

Building and Publishing Images

Build image

# Build image with tag 'minha-app:1.0'
docker build -t minha-app:1.0 .
# The dot (.) indicates Dockerfile is in current folder

# Use Dockerfile in different location
docker build -f caminho/Dockerfile -t minha-app .

# View layers and image size
docker history minha-app:1.0

Publish to Docker Hub

# Login to Docker Hub
docker login

# Tag image with your Docker Hub username
docker tag minha-app:1.0 seuusuario/minha-app:1.0

# Push to Docker Hub
docker push seuusuario/minha-app:1.0

Docker Compose

Tool to manage containers. Replaces docker run commands. Write everything in a YAML file and start services together.

docker-compose.yml Structure

A Compose file has four main sections: version, services, volumes, and networks.

docker-compose.yml

version: '3.8'

# ─── SERVICES ─────────────────────────────────────────────────
services:

  # Each service is a container
  nome-do-servico:
    image: imagem:tag          # OR
    build:                     # Build from Dockerfile
      context: .               # Folder with Dockerfile
      dockerfile: Dockerfile   # File name (if not 'Dockerfile')

    container_name: meu-app   # Fixed container name

    ports:
      - "HOST:CONTAINER"      # Port mapping

    environment:               # Environment variables
      - VARIAVEL=valor
      DATABASE_URL: postgres://user:senha@db/meudb

    env_file:                  # Or load from .env file
      - .env

    volumes:                   # Volumes and bind mounts
      - dados-nomeados:/var/dados
      - ./pasta-local:/app

    depends_on:               # Startup order
      db:                     # Wait for 'db' to be 'healthy'
        condition: service_healthy

    restart: unless-stopped   # Restart policy

    networks:                 # Networks to join
      - minha-rede

    healthcheck:              # Health check
      test: ["CMD", "curl", "-f", "http://localhost:3000/health"]
      interval: 30s
      timeout: 10s
      retries: 3
      start_period: 40s


# ─── NAMED VOLUMES ─────────────────────────────────────────
volumes:
  dados-nomeados:            # Volume managed by Docker
  outro-volume:
    external: true           # Externally created volume


# ─── NETWORKS ────────────────────────────────────────────────
networks:
  minha-rede:
    driver: bridge

Docker Compose Commands

Start services

# Start all services in background
docker compose up -d

# Start and rebuild images (when Dockerfile/code changed)
docker compose up -d --build

# Start only specific service
docker compose up -d nome-do-servico

Stop services

# Stop containers (keeps volumes and networks)
docker compose stop

# Stop and REMOVE containers and networks (volumes remain)
docker compose down

# Remove everything including volumes (CAREFUL: deletes data!)
docker compose down -v

Monitor

# List project containers
docker compose ps

# View logs from all services
docker compose logs

# Follow logs in real-time
docker compose logs -f

# Logs from specific service
docker compose logs -f app

Execute commands

# Open terminal in a service
docker compose exec app bash

# Execute one-off command
docker compose exec db psql -U usuario -d meudb

# Run one-off service (without starting others)
docker compose run --rm app npm test

Other commands

# View final resolved configuration (variables substituted)
docker compose config

# Rebuild service without stopping others
docker compose up -d --build app

# Scale a service (create multiple instances)
docker compose up -d --scale app=3

Complete Example — Node.js + PostgreSQL + Redis

docker-compose.yml

version: '3.8'

services:

  # ─── APPLICATION ───────────────────────────────────────────────
  app:
    build:
      context: .
      dockerfile: Dockerfile
      target: production      # Multi-stage build stage
    ports:
      - "${PORT:-3000}:3000"
    environment:
      - NODE_ENV=production
      - DATABASE_URL=${DATABASE_URL}
      - REDIS_URL=${REDIS_URL}
    depends_on:
      db:
        condition: service_healthy
      redis:
        condition: service_healthy
    restart: unless-stopped
    networks:
      - app-net

  # ─── DATABASE ──────────────────────────────────────────
  db:
    image: postgres:15-alpine
    env_file: .env
    volumes:
      - postgres-data:/var/lib/postgresql/data
      - ./scripts/init.sql:/docker-entrypoint-initdb.d/init.sql
    restart: unless-stopped
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U $${POSTGRES_USER} -d $${POSTGRES_DB}"]
      interval: 10s
      timeout: 5s
      retries: 5
      start_period: 30s
    networks:
      - app-net

  # ─── CACHE ───────────────────────────────────────────────────
  redis:
    image: redis:7-alpine
    command: redis-server --appendonly yes --maxmemory 256mb
    volumes:
      - redis-data:/data
    restart: unless-stopped
    healthcheck:
      test: ["CMD", "redis-cli", "ping"]
      interval: 10s
      timeout: 5s
      retries: 5
    networks:
      - app-net

  # ─── REVERSE PROXY ───────────────────────────────────────────
  nginx:
    image: nginx:alpine
    ports:
      - "80:80"
      - "443:443"
    volumes:
      - ./nginx.conf:/etc/nginx/nginx.conf:ro
      - certbot-data:/etc/letsencrypt
    depends_on:
      - app
    restart: unless-stopped
    networks:
      - app-net

volumes:
  postgres-data:
  redis-data:
  certbot-data:

networks:
  app-net:
    driver: bridge

docker-compose.override.yml

# docker-compose.override.yml
# This file is automatically merged with docker-compose.yml
# Use for development configs

version: '3.8'

services:
  app:
    build:
      target: development     # Use dev stage from Dockerfile
    volumes:
      - .:/app                # Bind mount for hot reload
      - /app/node_modules     # Preserve container's node_modules
    environment:
      - NODE_ENV=development
    command: npm run dev      # Override Dockerfile CMD

  db:
    ports:
      - "5432:5432"          # Expose port to connect externally

  redis:
    ports:
      - "6379:6379"

Cleanup

Cleanup commands permanently delete data. Always check what will be removed before confirming.

System cleanup

# View Docker disk usage
docker system df

# General cleanup (removes unused containers, networks, images, build cache)
docker system prune -a

# Remove all stopped containers
docker container prune

# Remove unused images
docker image prune -a

# Remove unused volumes
docker volume prune

# Remove unused networks
docker network prune

Development Tools

​Quick Reference

​Core Concepts

Docker

Image

Container

Volume

​Dockerfile

​Docker Run

​Managing Containers

​Managing Images

​Volumes

​Dockerfile

​Docker Compose

​Cleanup

Quick Reference

Core Concepts

Dockerfile

Docker Run

Managing Containers

Managing Images

Volumes

Dockerfile

Docker Compose

Cleanup