Introduction to Docker and Containers - Overview, Installation, Image and Container creation

Docker is an open source software used to develop and deliver the software in the form of packages known as Containers. Software used for hosting containers is known as Docker Engine. Docker is written in the Go programming language.

Containers are isolated from each other. Each container has its own libraries, configuration files and there are mechanisms through containers to communicate with each other. These containers are created from images which can be modified or customized based on the need. All the containers share the host operating system and hence these are light-weight.

An Image from which containers are created is nothing but a package of code, libraries, environment variables and configuration files.

For knowing differences between docker containers and virtual machines, click here. Here we learn how to install Docker, build an image, create containers using the image, clearing and viewing containers in the Linux operating system.

Docker provides two editions to use the software. One is free Docker Community Edition (CE) and another is Docker Enterprise Edition (EE).

Docker CE & EE features

Docker CE is free where Docker EE is a paid service. Docker CE provides container engine, built-in orchestration, networking and security purposes. This edition is used by individuals or small teams to experiment with apps built using containers.

Docker EE is for the enterprise development and IT industries who will be developing business-critical applications. It has the following features in addition to features of the Community edition.

• Scanning of Images for security reasons
• Image management as well as container app management
• Provides certified infrastructure and plugins
• Also provides Independent Software Vendor Containers

Installing Docker:

For learning purpose, we will be using Docker CE. Below are the steps to be followed for installing Docker CE in the Linux Operating system and Ubuntu distribution.

Note: To remove the docker if already present in your system use this command sudo apt-get remove docker docker-engine docker.io containerd runc

1. First, update the package list in your system using command sudo apt-get update
2. To download software repositories via HTTPS, run the command sudo apt-get install     apt-transport-https ca-certificates  curl     gnupg-agent software-properties-common
3. Add dockers GPG key to your system using command curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
4. Verify that the key is added to the system using command sudo apt-key fingerprint 0EBFCD88
5. To add the docker repository to system use sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable" 
6. Update the apt package list using sudo apt-get update
7. To install the latest version of docker use sudo apt-get install docker-ce docker-ce-cli containerd.io

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Verification of Docker Installation:

After installing the docker, follow the below steps to ensure that Docker CE is installed properly. 

• To check the version of docker, use docker version command. It gives the version details of the API, built date etc. details which means docker is installed
• Another way to verify the docker installation is to run hello-world image using sudo docker run hello-world command which is pulled from the docker library and prints Hello from Docker!

Building Image using Docker:

To build an image using Docker tool, we need to create Dockerfile and have the code at the same path where Dockerfile is present.

Consider hello.py which prints “Hello World, Welcome to Docker” is as below:

print("Hello-World")

print("Welcome to Docker")

The sample Dockerfile for executing hello.py file in the container is

FROM ubuntu:16.04 

#Base Image

COPY hello.py /root

#Copy hello.py to root directory in Container

RUN chmod +x /root/hello.py 

#Set executable permission for hello.py

RUN apt-get update

#Update Package list in container

RUN apt-get install python -y 

#Install Python in Container

The Dockerfile and hello.py should be in the same path. Go to that path and run sudo docker build --tag=hello . Here in this command hello is the name of the image to be created.

To create a container with a random name and execute hello.py using python /root/hello.py inside the container, use sudo docker run -it hello bash. Here in this command hello is the name of the image.

Then execute hello.py which displays the welcome message.