Docker: containerize your applications

Introduction

Hey there! Let's talk about Docker - it's this cool open-source platform that makes deploying apps a breeze through something called containerization. Think of containers as neat little packages that bundle up your application with everything it needs - the code, runtime, tools, and libraries. This way, your app runs the same way no matter where you deploy it.

What makes Docker special? Unlike traditional virtual machines that simulate an entire operating system, Docker containers share the host's kernel and only isolate the app processes. This makes them super lightweight and quick to start up - perfect when you need to get things running fast!

Here's why so many developers love Docker:

• Consistency: Your app works the same way everywhere - from your laptop to the production server
• Isolation: Apps and their dependencies stay in their own sandbox, away from your host system
• Portability: Got Docker? Then you can run the containers - on Mac, Windows, Linux, in the cloud, wherever!
• Efficiency: Containers are lightweight and share the host kernel, so they use way fewer resources than VMs
• Scalability: Need more power? Just spin up more containers - it's that simple
• Versioning: Made a mistake? No problem! You can track changes and roll back to previous versions

Installation

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sudo apt-get update && apt-get install -y ca-certificates curl gnupg lsb-release

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sudo mkdir -p /etc/apt/keyrings
curl -fsSL https://download.docker.com/linux/debian/gpg | sudo gpg --dearmor -o /etc/apt/keyrings/docker.gpg

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echo "deb [arch=$(dpkg --print-architecture) signed-by=/etc/apt/keyrings/docker.gpg] https://download.docker.com/linux/debian $(lsb_release -cs) stable" | sudo tee /etc/apt/sources.list.d/docker.list > /dev/null

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sudo apt-get update
sudo apt-get install -y docker-ce docker-ce-cli containerd.io docker-compose-plugin

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sudo yum install -y yum-utils

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sudo yum-config-manager --add-repo https://download.docker.com/linux/centos/docker-ce.repo

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sudo yum install -y docker-ce docker-ce-cli containerd.io docker-compose-plugin

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sudo systemctl start docker
sudo systemctl enable docker

Verify the installation:

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sudo docker run hello-world

Configuration

Memory Management

For optimal container performance, especially when running memory-intensive applications, you'll need to enable proper memory management by modifying your boot parameters:

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# Edit GRUB configuration
sudo nano /etc/default/grub

# Add these parameters to GRUB_CMDLINE_LINUX_DEFAULT
GRUB_CMDLINE_LINUX_DEFAULT="quiet cgroup_enable=memory swapaccount=1"

# Update GRUB and reboot
sudo update-grub
sudo reboot

Storage Location

By default, Docker stores its images and containers in /var/lib/docker. If you want to change this location (for example, to a larger drive), you can modify Docker's configuration:

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# Edit Docker daemon configuration
sudo nano /etc/docker/daemon.json

# Add or modify the storage-driver and data-root settings
{
  "storage-driver": "overlay2",
  "data-root": "/path/to/your/docker/data"
}

# Restart Docker service
sudo systemctl restart docker

Alternatively, if you're using Docker installed via package manager, you can edit:

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sudo nano /etc/default/docker

# Add or modify DOCKER_OPTS
DOCKER_OPTS="-g /path/to/your/docker/data"

# Restart Docker
sudo systemctl restart docker

Usage

Basic Commands

Pulling Images

Download Docker images from Docker Hub:

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docker pull ubuntu:latest
docker pull nginx:1.21

List Images

View all downloaded images:

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docker images

Run Containers

Start a container from an image:

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docker run -it ubuntu bash    # Interactive terminal
docker run -d nginx           # Detached mode

List Containers

View running containers:

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docker ps           # Running containers
docker ps -a        # All containers (including stopped)

Stop and Remove Containers

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docker stop <container_id>
docker rm <container_id>

Expose Ports

Map container ports to host ports:

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docker run -d -p 8080:80 nginx

This maps port 80 in the container to port 8080 on the host.

Mount Volumes

Share data between host and container:

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docker run -d -v /host/path:/container/path nginx

For named volumes:

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docker volume create my_volume
docker run -d -v my_volume:/container/path nginx

Network Configuration

Create a Network

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docker network create my_network

Connect Containers

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docker run -d --name db --network my_network mysql
docker run -d --name web --network my_network nginx

Build Custom Images

1. Create a Dockerfile:

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FROM ubuntu:20.04
RUN apt-get update && apt-get install -y nginx
COPY ./app /var/www/html
EXPOSE 80
CMD ["nginx", "-g", "daemon off;"]

1. Build the image:

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docker build -t my-app:1.0 .

Advanced Container Management

Committing Changes

After making changes to a container, you can create a new image that preserves those changes:

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# Make changes to a container
docker exec -it <container_id> bash
# ... make your changes ...

# Commit changes to a new image
docker commit <container_id> username/my-custom-image:tag

# View your new image
docker images

This is particularly useful for creating custom images based on your modifications.

Viewing Container Differences

To see what files have been changed, added, or deleted in a container compared to its base image:

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docker diff <container_id>

The output uses these prefixes:

• A: Added
• D: Deleted
• C: Changed

This is extremely helpful for troubleshooting and understanding what modifications have occurred inside a container.

Container Management

View Container Logs

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docker logs <container_id>

Execute Commands in Running Containers

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docker exec -it <container_id> bash

Check Container Resource Usage

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docker stats <container_id>

References

• Docker Official Documentation
• Docker Hub