Docker Installation on CentOS 7

Registry Mirror Configuration

To improve image download speed, configure Alibaba's registry mirror by editing /etc/docker/daemon.json:

{
  "registry-mirrors": ["https://91cntlkt.mirror.aliyuncs.com"]
}

Restart the Docker service after making changes:

systemctl restart docker

Installling Docker

Install Docker using yum:

yum install -y docker

Start and enable Docker at boot:

systemctl start docker
systemctl enable docker

Basic Docker Commands

Check Docker status:

docker --help
docker info

List images and containers:

docker images
docker ps -a

Pulling Base Images

Download CentOS and Tomcat images:

docker pull centos
docker pull tomcat

Running Containers

Run Tomcat container with port mapping:

# Detached mode
docker run -d -p 8081:8080 tomcat:latest

Container Management

Restart a running container:

docker restart container_name

Remove a image:

docker rmi -f image_id

File Transfer Between Host and Container

Copy files from container to host:

docker cp container_name:/path/to/file /host/path/

Copy files from host to container:

docker cp /host/path/file container_name:/path/to/destination/

Kernel Requirements

Verify kernel version:

uname -r

For CentOS 6.5 systems, upgrade kernel:

rpm --import https://www.elrepo.org/RPM-GPG-KEY-elrepo.org
rpm -ivh http://www.elrepo.org/elrepo-release-6-5.el6.elrepo.noarch.rpm
yum -y --enablerepo=elrepo-kernel install kernel-lt

Edit /etc/grub.conf to set default kernel:

# Change default=1 to default=0

Reboot system to apply changes.

Installing Software in Container

Run interactive container:

docker run -it centos:latest /bin/bash

Inside container, install JDK and Tomcat:

# Extract JDK
mkdir -p /opt/java
tar -zxf jdk-7u55-linux-i586.tar.gz -C /opt/java

# Rename JDK directory
mv /opt/java/jdk1.7.0_67 /opt/java/jdk

# Extract Tomcat
wget http://archive.apache.org/dist/tomcat/tomcat-7/v7.0.52/bin/apache-tomcat-7.0.52.tar.gz
tar -zxf apache-tomcat-7.0.52.tar.gz -C /opt
mv /opt/apache-tomcat-7.0.55 /opt/tomcat

Set environment variables:

# Add to ~/.bashrc
export JAVA_HOME=/opt/java/jdk
export PATH=$PATH:$JAVA_HOME/bin

Apply environment settings:

source ~/.bashrc

Install missing dependencies if needed:

yum install -y glibc.i686

Creating Startup Script

Create script to launch Tomcat:

#!/bin/bash
source ~/.bashrc
sh /opt/tomcat/bin/catalina.sh run

Make executable:

chmod +x /root/run.sh

Building Custom Image

Commit container changes to new image:

docker commit container_id username/image_name:tag

Running Java Web Application

Launch container with custom image:

docker run -d -p 58080:8080 --name java-web username/image_name:tag /root/run.sh

Docker Concepts

Images vs Containers

Docker images are read-only templates used to create containers. Containers are running instances of images with a writable layer added on top.

Layered Architecture

Image are built in layers where each layer represents a set of changes. The base layer is the kernel, followed by base OS, then application layers.

Container Lifecycle

Containers can be in running or stopped states. Changes made during runtime are saved to the container's writable layer, not the base image.