Docker

Enable Docker Deamon

On Linux

Check status

sudo systemctl status docker

Activate docker

sudo systemctl start docker

Add User To The Docker Group

To avoid using sudo every time

sudo usermod -aG docker $USER

Starting The Container

With Dockerfile

docker build -t my-express-app . # create an image
docker run -p 5000:5000 my-express-app # start the container

Pros:

• Simple if you’re only running one container.

Cons:

• You must manually build the image before running it.
• If you add MongoDB later, managing multiple containers gets trickier.

With Compose File

If an image already exists, that will be used

docker-compose up -d creates the image and starts the container

Pros:

• No need to manually build the image.
• Everything starts with a single command.
• Easier container networking (backend & database talk to each other automatically).

Cons:

• Slightly more setup (the yml file), but worth it if you use multiple containers.

Stopping The Container

When Started With A Dockerfile

docker ps  # shwo running containers and get the container id
docker stop < container_id >  # stop container

When Started With Compose File

docker-compose down // stops and removes containers

Stop containers without removing them

docker-compose stop

Stop and remove containers and all images (created with that compose file)

docker-compose down --rmi all

Generic Commands

Check if docker is running

docker info

Show running containers

docker ps

Show images (images are not displayed in the project folder, instead in a different docker storage place)

docker images

Remove Container

docker rm <container_id>

Remove Image

docker rmi my-express-app

Show docker disc usage

docker system df

Save docker image in a file

docker save -o <image-name>.tar <image-name>

Import docker image from a file

docker load -i <image-name>.tar

Clean up docker container/images

docker system prune

# Removes all stopped containers
# Removes all unused networks
# Removes all dangling images (not tagged or referenced by any container.)

docker system prune --volumes

# Additionally removes all volumes

Sample Docker File

Dockerfile (no file extension like .txt)

# alpine is a lightweight linux distribution
FROM node:18-alpine 
# the working directory inside the container
WORKDIR /app 
# copies package.json and package-lock.json to the container
COPY package*.json ./ 

RUN npm install
# copies all other files (except those in .dockerignore) to the container
COPY . . 
# container will listen on port 5000, but it's only for documentation
EXPOSE 5000 
# executes node server.js
CMD ["node", "server.js"] 

Sample Compose File

Configures a nodejs backend and a mongodb server

docker-compose.yml

version: '3.9' # Specifies the Docker Compose file format version

networks: # Defines a custom network named app-network
  app-network: # All services in this file will communicate through this network

services: # Each service represents a containerized application
  backend: # Defines a service named backend
    build: . # Tells Docker to build the image from the current directory
    container_name: wiki-backend # Assigns a custom container name wiki-backend to the backend container
    ports:
      - "5000:5000" # Maps port 5000 on the host to port 5000 on the container
    networks:
      - app-network # Connects the backend service to the app-network
    depends_on:
      - mongo  # Ensures MongoDB starts before the backend

  mongo:
    image: mongo:latest # Uses the official MongoDB image from Docker Hub
    container_name: mongo-db
    ports:
      - "27017:27017"
    networks:
      - app-network
    volumes: # Defines a named volume mongo-data for persistent storage of MongoDB data
      - mongo-data:/data/db 

volumes: # Even if the MongoDB container is deleted, the data remains stored on the host machine
  mongo-data:

Run PostgreSQL From Compose File

version: '3.8'
services:
  postgres:
    image: postgres
    container_name: postgres-container
    restart: always
    environment:
      POSTGRES_USER: myuser
      POSTGRES_PASSWORD: mypassword
      POSTGRES_DB: mydatabase
    ports:
      - "5432:5432"
    volumes:
      - pgdata:/var/lib/postgresql/data

volumes:
  pgdata:

Connect using psql from inside the Docker container

docker exec -it postgres-container psql -U myuser -d mydatabase