Docker Image

Docker image can be described as a template with all required configurations Whereas a container is a  running instance of Docker image. Like containers, image are not bound to the states i.e. Image does not have states.

There are different images available from the OS/Application vendor along with the custom images from the community.

When working on container a DevOps/Application engineer generally create their own Docker image with all the customization, this enable them to launch a container quickly.

Methods for custom image creation

Interactive Method:

In this way, you can download the base Docker OS image -> create container -> manually launch a shell -> perform the customization -> commit the changes.

This process will save your container to a Docker image and that image can be stored/distributed.

Automated  method using Dockerfile:

Dockerfile is text file with the directives/instructions for the image creation. “docker build” command is used to build the image which creates/configures the image automatically by reading Dockerfile. Dockerfile accept the in the following format

DIRECTIVE               arguments

In the last session on “Docker”, we have covered the “interactive way” of image creation and in this session we are going to create a Docker image using the Dockerfile method.

LAB

Using the Dockerfile, we are going to create an Apache HTTPD Web server on CentOS 7 image, At high level below configuration will be performed/applied to the Docker image.

• Download the official CentOS 7 image.
• Perform package update on the image.
• Install Apache HTTP server.
• Add a directive to include/copy the index.html from Docker mgmt server to document root ( i.e. /var/www/html ) of the image.
• Enable port 80 for automatically whenever a container created from this image.
• Configure the auto start-up of Apache HTTPD service.

Below are the directives we are going to use in the Dockerfile.

FROM :   this directive tells which base image to be used to create the custom image, example centos/ubuntu etc.

RUN    :       this directive is use to define the command to be executed during the image build.

ADD    :      this directive is used to defines the files/directories to be copied from the source (local server) to the image during image build.

ENTRYPOINT :     this directive defines container as executable.

CMD :             this directive is used to define the arguments for the ENTRYPOINT command.

EXPOSE :  this directive defines the network ports on which container will listen.

Sample Dockerfile

#  use latest centos7 image

FROM centos:latest

#  add the image maintainer name and email id

MAINTAINER Aghassi email: [email protected]

# update the centos image with latest available updates

RUN yum update -y

RUN yum clean all

# install network utilities, such as ( ifconfig, netstat, etc)

RUN yum install net-tools -y

# install apache httpd web server

RUN yum install httpd -y

RUN yum clean all

# copy the index.html file from current directory to image's document root

ADD index.html /var/www/html/

# define image to allow listen on port 80 (whenever a container created)

EXPOSE 80

# define the commands to be executed when container boots (created from this image)

ENTRYPOINT [ "/usr/sbin/httpd" ]

CMD [ "-D",  "FOREGROUND" ]

Bulid image

# docker build -t [repository/image_name]:[tag] .

Test the newly created image by creating a container

# docker run -it -d -P [image id]

#curl [container IP]:80

Why to use custom Network Subnet for Docker Networking?

Docker container makes use of default subnet "172.17.0.0/16" for Networking. There may be many scenarios where we can’t use the default network due to some restrictions or in case subnet already used in the network.

Lab Tasks

In this quick session, we will change the network from default subnet "172.17.0.0/16" to "10.10.10.10/24". The bridge interface is remain to docker0 i.e. default.

Configure the Custom Network

Stop The Docker Service

# systemctl stop docker.service

Bring down the Docker bridge docker0

# ip link set dev docker0 down

Verify if IP forwarding is enabled, if not enable it in sysctl.conf

# sysctl net.ipv4.conf.all.forwarding

Update new subnet in the /etc/sysconfig/docker-network add the following to DOCKER_NETWORK_OPTIONS:

"--bip=YOUR>CIDR>ADDRESS/24"

Example

DOCKER_NETWORK_OPTIONS="--bip=10.10.10.10/24"

Remove default subnet’s MASQUERADE rules from the POSTROUTING chain in network iptables:

# iptables -t nat -F POSTROUTING

# iptables -t nat -F DOCKER

Start Docker service:

# systemctl start docker.service

Verify that the MASQUERADE rule have new subnet added to the POSTROUTING chain:

# iptables -t nat -L -n

Validation

Check the new subnet is on the bridge now:

# docker network inspect bridge

Check IP Address of the Container

# docker inspect -f '{{ .NetworkSettings.IPAddress }}' [Container ID]

Run a docker container and check container have

# docker run -it [Container Name] /bin/bash

What is Docker?

Docker is an open platform for developers and system engineers to build, ship, and run distributed applications, whether on Bare Metal System (Physical), VMs, or the Cloud, Docker is not a container technology like Xen/KVM etc.

Docker provides an additional layer abstraction and automation of operating system I virtualization on Linux.

Advantages of using Docker

Portability – In Docker system, an application and its prerequisites/dependencies can be bundled into a single container/image, whish will be independent of host kernel, can be easily ported to different system.

Quick Application Deployment – As application and its dependencies can be bundled into single images, it makes easy to quickly deploy the apps.

Sharing – You can share your Docker image with other using remote repositories.

Lightweight – Docker images have very small, they need very low compute capacity and storage, …

Easy Maintenance – Maintenance is very quick and easy.

Cost Saving – Open Source technology and don’t need heavy compute.

Docker Containers vs. Virtual Machines

• Docker container can be created/destroyed very quickly as compare to the virtual machines.
• Docker containers are light weight is compare to the virtual machines. Being lightweight more containers can run at some time on a host.
• Docker container make use of resources very efficiently. In case of virtual machines capacity is need to be reserved (compute + storage), whereas this is not needed is case of Docker containers.
• Virtual Machines can be migrated across servers when they are running, but Docker need to stopped before migration as there is no hypervisor layer.

 

*Images taken from Docker Documentations

Docker Terminologies

• Images – Images are templates for the docker containers.
• Containers – created from Docker images and run the actual application.
• Docker Daemon – The background service running on the host that manages building, running the containers.

Prerequisites Docker Installation

1. CentOS 7 64 Bit / Kernel 3.10.x kernel in the minimum required.
2. Disabled the SELinux and FirewallD services:  # systemctl stop firewalld
3. Install EPEL repository:    # yum install -y epel-release

 Install Docker via yum provided by CentOS (method 1)

# yum install -y docker

Install Docker CE (community edition)  Software (method 2)

First remove older version of docker (if any):

# yum remove docker docker-client docker-common docker-selinux docker-engine-selinux docker-engine docker-ce

Next install needed packages:

# yum install yum-utils device-mapper-persistent-data lvm2 -y

Configure the docker-ce repo:

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

Finally install docker-ce:

# yum install docker-ce -y

Enable and Start Docker service

# systemctl enable docker
# systemctl start docker

How to find out info about Docker network bridge and IP addresses

Default network bridge named as docker0 and is assigned with an IP address. To find this info run the following:

# ip a

# ip a s docker0

How to run docker commands

The syntax is:

# docker command
# docker command arg
# docker [options] command arg
# docker help | more

Getting help

# docker help | more

Run ‘docker COMMAND --help‘ for more information on a command:

# docker ps --help
# docker cp --help

Check the Docker version

# docker version

Check Detailed Docker Information

# docker info

How to test your docker installation

Docker images are pulled from docker cloud/hub such as docker.io or registry.access.redhat.com and so on. Type the following command to verify that your installation working:

# docker run hello-world

Search Docker Images on Internet

Now you have working Docker setup. It is time to find out images. We can find images for all sort of open source projects and Linux distributions. To search the Docker Hub/cloud for centos or nginx image run:

# docker search centos

# docker search nginx

Download Docker Images

To pull an image named centos or nginx from a registry, run:

# docker pull centos:centos7

# docker pull nginx

To Display the list of locally available images

# docker images

TAG − This is used to logically tag images.
Image ID − This is used to uniquely identify the image.
Created − The number of days since the image was created.
Virtual Size − The size of the image. 

Remove Docker Image

When you have lots of running which are obsolete or you no longer need any Docker image then you can remove that image using the following command.

# docker rmi [IMAGEname]

To test your new image

The concept is little catchy, whenever a command is sent for execution in the Docker image, a container in obtained. When this command execution is finished, the container gets stopped (a non-running or exited container state). It Means at every command execution into the same image a new container is created again and again and exited.

# docker run centos:centos7 /bin/ping 1it.click -c 5

List Docker Containers

Whenever a command execution is performed on a Docker Image a container is created and gets stopped after execution but it remains in exited or non-running state. The following command will display a list of the running and stopped (non-running) containers:

# docker ps -l

In a production environment there are many running containers and to list them we have command. This command is used to get the currently running containers:

# docker ps

This Can also be used with -a argument and this command will list all of the containers on the system:

# docker ps -a

Checking Docker Networking

# docker network ls

# docker network inspact [network name]

Checking Resource Consumption by Running Container

# docker stats

Check Resource limits for a docker container

# docker run -it -c 256 -m 300M centos:centos7 /bin/bash

Stop/Start/Restart operation

# docker start [container ID]                ## to start a docker container

# docker stop [container ID]                 ## to stop a docker container

# docker restart [container ID]              ## to restart a docker container

Committing the Docker Container Updates (This command turns your container to an image) And Adding a Repository/Tag value to a image

# docker commit [container ID]

# docker tag [image ID] <repo : tags>

Removing/Deleting a container

# docker rm [container ID]

Checking the docker container Logs

# docker logs [container ID]

Lets create our container and hots a demo website quickly using Python Simple HTTP Server module quickly will listen on port 8080:

# mkdir -p /var/www/html

# echo "This is my Aghassi's test Docker Website" > /var/www/html/demowebpage.txt

# docker run -d -p 8080:8080 --name="python_web" -v /usr/sbin:/usr/sbin -v /usr/bin:/usr/bin -v /usr/lib64:/usr/lib64 -w /var/www/html -v /var/www/html:/var/www/html centos:centos7 /bin/python -m SimpleHTTPServer 8080

-d, –detach                             Run container in background and print container ID

-p, –publish list                     Publish a container’s port(s) to the host (default [])

-v, –volume list                     Bind mount a volume (default [])

-w, –workdir string              Working directory inside the container

Check the network ports allocation:

# ss -tupln |grep 8080

Lets test the website:

# curl localhost:8080/demowebpage.txt

How to run Docker nginx image

Now you pulled nginx image, it is time to run it:

# docker run --name my-nginx-i --detach nginx

Say you want to host simple static file hosted in /var/www/html/ using nginx container:

# docker run --name my-nginx-ii -p 80:80 -v /var/www/html/:/usr/share/nginx/html:ro -d nginx

Where,

–name my-nginx-i : Assign a name to the container
–detach : Run container in background and print container ID
-v /var/www/html/:/usr/share/nginx/html:ro : Bind mount a volume
-p 80:80 : Publish a container’s port(s) to the host i.e redirect all traffic coming to port 80 to container traffic

Go ahead and create a file named index.html in /var/www/html/:

# echo 'Welcome. I am Nginx server locked inside Docker' > /var/www/html/index.html

Test it:

curl http://your-host-ip-address/
curl 192.168.1.7

Sample outputs:

Welcome. I am Nginx server locked inside Docker

How to run a command in a running container

Run ls /etc/nginx command for my-nginx-i container

# docker exec e535e4c08c07 ls /etc/nginx

OR

# docker exec my-nginx-i ls /etc/nginx

Want to gain bash shell for a running container and make changes to nginx image?

# docker exec -i -t e535e4c08c07 bash

OR

# docker exec -i -t my-nginx-i bash