Docker Installation and Usage

Add the Docker CE Repository

Docker provides an official repository for enterprise Linux. The following steps apply to RHEL, CentOS Stream, AlmaLinux, Rocky Linux, and other EL-based distributions.

EL 10 / EL 9 / EL 8

# Install the DNF plugin that provides config-manager
sudo dnf install -y dnf-plugins-core

# Add the official Docker repository
sudo dnf config-manager --add-repo \
  https://download.docker.com/linux/centos/docker-ce.repo

CentOS 7 (EOL, reference only)

sudo yum install -y yum-utils

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

Note

If you are in mainland China, Docker Hub and the official repository may be slow to access. Consider using a domestic mirror source for the repository URL or configuring a Docker registry mirror.

Install Docker

1. Install Docker Engine, CLI, and container runtime:

   EL 10 / EL 9 / EL 8

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

   CentOS 7 (EOL, reference only)

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

2. Start Docker and enable it at boot:

   sudo systemctl enable --now docker

3. Verify the installation:

   sudo docker version
   sudo docker run hello-world

4. (Optional) Add the current user to the docker group to avoid using sudo every time:

   sudo usermod -aG docker $(whoami)

   Log out and log back in for the change to take effect.

Caution

Adding a user to the docker group is equivalent to granting that user root privileges, since the Docker daemon runs as root. Use caution in production environments.

Basic Usage

Pull Images

# Pull images from Docker Hub
docker pull nginx:latest
docker pull almalinux:9

# Search for images
docker search mariadb

# View local images
docker images

Run Containers

# Run in the foreground, automatically remove on exit
docker run --rm -it almalinux:9 /bin/bash

# Run Nginx in the background
docker run -d --name my-nginx -p 80:80 nginx:latest

# Run MariaDB with environment variables and volume mounts
docker run -d --name my-db \
  -p 3306:3306 \
  -e MYSQL_ROOT_PASSWORD=your_password \
  -v /data/mysql:/var/lib/mysql \
  mariadb:10.11

View Containers

# View running containers
docker ps

# View all containers
docker ps -a

# View container logs
docker logs my-nginx
docker logs -f --tail 100 my-nginx

# View container details
docker inspect my-nginx

# View container resource usage
docker stats

Stop and Remove

# Stop a container
docker stop my-nginx

# Start a stopped container
docker start my-nginx

# Restart a container
docker restart my-nginx

# Remove a stopped container
docker rm my-nginx

# Force remove a running container
docker rm -f my-nginx

# Remove all stopped containers
docker container prune

# Remove unused images
docker image prune -a

Enter a Container

# Enter a container with an interactive terminal
docker exec -it my-nginx /bin/bash

# Execute a command inside a container
docker exec my-nginx nginx -t

Docker Compose

Docker Compose is used to define and manage multi-container applications. Docker CE includes the docker compose subcommand (V2) out of the box.

Write a Compose File

Create docker-compose.yml:

services:
  web:
    image: nginx:latest
    ports:
      - "80:80"
      - "443:443"
    volumes:
      - ./nginx.conf:/etc/nginx/nginx.conf:ro
      - ./html:/usr/share/nginx/html:ro
      - certbot_data:/etc/letsencrypt:ro
    depends_on:
      - app
    restart: unless-stopped

  app:
    build: ./app
    expose:
      - "8000"
    environment:
      - DATABASE_URL=mysql://root:changeme@db:3306/myapp
    depends_on:
      - db
    restart: unless-stopped

  db:
    image: mariadb:10.11
    environment:
      MYSQL_ROOT_PASSWORD: changeme
      MYSQL_DATABASE: myapp
    volumes:
      - db_data:/var/lib/mysql
    restart: unless-stopped

  redis:
    image: redis:7-alpine
    restart: unless-stopped

volumes:
  db_data:
  certbot_data:

Manage Compose Services

# Start all services (in the background)
docker compose up -d

# View service status
docker compose ps

# View logs
docker compose logs -f

# View logs for a specific service only
docker compose logs -f app

# Rebuild and start
docker compose up -d --build

# Stop all services
docker compose down

# Stop and remove data volumes
docker compose down -v

# Scale service instances
docker compose up -d --scale app=3

Build Custom Images

Write a Dockerfile

FROM almalinux:9-minimal

RUN microdnf install -y python3 python3-pip && \
    microdnf clean all

WORKDIR /app
COPY requirements.txt .
RUN pip3 install --no-cache-dir -r requirements.txt

COPY . .

EXPOSE 8000

USER 1001

CMD ["python3", "-m", "uvicorn", "main:app", "--host", "0.0.0.0", "--port", "8000"]

Build and Push

# Build the image
docker build -t my-app:v1 .

# Tag the image
docker tag my-app:v1 registry.example.com/my-app:v1

# Push to a private registry
docker push registry.example.com/my-app:v1

Storage Driver Configuration

Docker uses the overlay2 storage driver by default on EL systems, which is the recommended option.

Check the Current Storage Driver

docker info | grep "Storage Driver"

Configure the Docker Daemon

Edit or create /etc/docker/daemon.json:

{
  "storage-driver": "overlay2",
  "data-root": "/data/docker",
  "log-driver": "json-file",
  "log-opts": {
    "max-size": "100m",
    "max-file": "3"
  },
  "registry-mirrors": [
    "https://mirror.example.com"
  ]
}

Restart Docker to apply the configuration:

sudo systemctl restart docker

Tip

The data-root option can move the Docker data directory to another partition to avoid filling up the root partition. For heavy Docker usage, it is recommended to allocate a dedicated disk for /data/docker.

Migrate the Storage Directory

To migrate Docker data from the default /var/lib/docker to a new location:

1. Stop Docker:

   sudo systemctl stop docker

2. Copy existing data:

   sudo rsync -aP /var/lib/docker/ /data/docker/

3. Set data-root in /etc/docker/daemon.json:

   {
     "data-root": "/data/docker"
   }

4. Start Docker and verify:

   sudo systemctl start docker
   docker info | grep "Docker Root Dir"

5. After confirming everything works, remove the old directory:

   sudo rm -rf /var/lib/docker

Firewall Configuration

Docker manages its own iptables rules. If you are using firewalld, note the following:

# Allow inter-container communication
sudo firewall-cmd --permanent --zone=trusted --add-interface=docker0
sudo firewall-cmd --reload

Caution

Docker directly manipulates iptables to publish ports, which may bypass firewalld rules. In production environments, consider setting "iptables": false in /etc/docker/daemon.json and manually managing firewall rules, or using an external firewall/security group to control access.

Common Maintenance Commands

# View Docker disk usage
docker system df

# Comprehensive cleanup of unused resources
docker system prune -a --volumes

# Export a container as an image
docker commit my-nginx my-nginx-backup:latest

# Export/import image files
docker save -o nginx-backup.tar nginx:latest
docker load -i nginx-backup.tar