Docker Step by Step

Sofiene Ben Khemis
Nov 13, 2020 · 11 min read

“Docker allows you to package an application with all of its dependencies into a standardized unit for software development.”

Before I tell you about Docker I will tell you a story that every developer has experienced at least once.

Once upon a time, there was a young developer who was quietly coding on his computer. He was in a hurry because he had to present his work the next morning. After hours of work, the application was there, and it worked perfectly! The next day, our coder arrived proudly for his presentation, with his project on a USB key. He transfers it to his friend’s computer and there, it doesn’t work!

What is the problem?

Our developer’s application doesn’t work on his friend’s computer because of an environmental problem. Between two systems, there may be version differences on dependencies or missing libraries.

Here, our problem is limited to 2 systems, but imagine a team of 10 persons with computers under Mac OS, Linux, or even Windows, a test server under Ubuntu, and a production server under CentOS. Making sure their application works well on all these environments can be a real nightmare!

But fortunately, there are solutions and among them we have Docker.

What is Docker?

So Docker can solve our environmental problem, because no matter which machine we use, the code will run the same way.

Installation and first hands-on

After that, you can check if docker is installed by running the commanddocker version

Run your first container

To run this image, docker tries to find the name of the image locally, if it exists it will launch it directly into a new container but if it doesn’t exist, Docker will launch the pull command docker pull hello-world that pulls the image from the docker hub and then it will run it into a new container.

To list all local Docker images you have just to run docker images

To delete an image use docker image rm REPOSITORY for example docker image rm hello-world

To list all the running containers you just need to run docker ps and to see all the containers even if they are stopped docker ps -a

To stop a running container you just need to run docker stop CONTAINER_ID

To delete a container you just need to run docker rm CONTAINER_ID and docker rm -f CONTAINER_ID to force deleting a running container. But I don’t recommend this. It’s better to stop the container before deleting it.

For more docker command details visit the Docker reference.

Docker Hub

When you click on any image you will find a page like this

You copy and paste the command docker pull nginx then docker will start the pull of the Nginx image to your local workspace.

By giving only the image name to the pull command docker will pull the latest existing version. So if you want to get a specific version you need to use Tags. For example, if you want the get version number 1.18 you run docker pull nginx:1.18

When the pull finish you can check you locale images using docker images You will find 2 images with the same name but with a different tag.

To run a specific image it is like the pull command you just need to add the tag after the image name docker run nginx:1.18 or docker run nginx to run the latest version.

You can see details about a docker image using the command docker inspect nginx

Dockerfile

Build your first image

Let’s create an image for a java program and run it inside a container.

I’ll use a simple java program that shows “This is java app using Docker” when you run it.

now let’s create our Dockerfile with the following instructions.

Once we have these two files, we are ready to build our image for the java program by just running docker build -t IMAGE-NAME:TAG . in our case, it will be docker build -t my-java-app:1.0 .

Now let’s see what images we have using docker images

We have our image my-java-app and another one called openjdk. Rember is the docker file we have FROM openjdk:8. Our image is based on another image.

We will need the openjdkimage every time we lunch a build for our java app.

Push your first image to the docker hub

Once your image is ready, you can push it to the docker hub. For this, you need to have a docker account. If you don’t have a Docker account, sign up for one here.

Then log into the Docker public registry on your machine usingdocker login -u USERNAME -P PASSWORD

Now you need to add a tag to your image before pushing it using this command docker tag my-java-app:1.0 bksofiene/my-java-app:1.0-Final and finally, run docker push bksofiene/my-java-app:1.0-Final

Now as you can see, my image is uploaded to the docker repository and it is accessible anywhere at any time. You can change the image restriction to make it private if you want.

For more details about the Dockerfile visit the Dockerfile reference.

Network containers

Container ports must be mapped to the ports of the host to avoid conflicts.

When Docker starts, it creates a virtual interface called docker0on the host machine with an IP address randomly allocated.

From Docker

To see existing networks you just need to run docker network ls

Docker by default have 3 networks:

  • Bridge: It is the Docker default networking mode which will enable the connectivity to the other interfaces of the host machine as well as among containers.
  • Host: In this mode container will share the host’s network stack and all interfaces from the host will be available to the container. The container’s hostname will match the hostname on the host system.
  • None: This mode will not configure any IP for the container and doesn’t have any access to the external network as well as for other containers.

Now if we want to create our own network we just need to run docker network create NETWORK_NAME

let’s do a little exercise to understand things better.

We are going to create an application with 3 containers in a private network :
- Web1: an unmapped Nginx server that should display Server ONE
- Web2: an unmapped Nginx server that should display Server TWO
- myhaproxy: a mapped haproxyserver, which will receive in FrontEnd the HTTP requests of the clients on port 80 of the Docker Host and which will redirect in BackEnd to the web servers.

HAProxy is a free, open-source high availability solution, providing load balancing and proxying for TCP and HTTP-based applications.

In our case, we will use it for a round-robin load balance.

I’ll start by creating 2 folders, Web1 and Web2 inside the first one, I’m going to create an HTML file called index.html that contains Server ONE and inside the second one I’ll create the same file but with Server TWO as a value.

inside each folder, we need to add the Dockerfile

Now let’s build images, if you are inside the Web1folder run docker build -t server1 . if you are inside the parent folder run docker build -t server1 web1 Same for the Web2folder docker build -t server2 . inside the folder or docker build -t server2 web2 inside the parent folder.

Now let’s create the haproxy image, first of all, we need to configure the haproxy.cfg

Then we create the Dockerfile

Now let’s build our image docker build -t myhaproxy myhaproxy

After that make sure you have all the 3 images and the web network.

Finally, we run our images:

docker run -d --network=web --name=web1 server1

docker run -d --network=web --name=web2 server2

docker run -d --network=web --name=haproxy -p 80:80 myhaproxy

-d : is for detach (run container in background), --network : to define our network, -p : for exposing port

Now ping curl 0 many times. you will get a response from a different container each time.

For more details about the docker network visit Docker networking.

Volumes

Volumes are the preferred mechanism for persisting data generated by and used by Docker containers.

From Docker

To create a volume, we will use the following command docker volume create test-volume and then use docker volume ls to list all existing volumes

To get details about the created volume use docker volume inspect test-volume

As shown in the Mountpoint value our data will be stored under /var/lib/docker/volumes/test-volume/_data

Now let’s try to get into this directory and create a text file called test.txt and put This is a test for a volume use case inside.

cat >> test.txt then type your text then close the input mode using ctrl d

Let’s go back to our work directory using cd ~ and create a docker image that accesses to the test.txt file and edit it.

Let’s create our Dockerfile

Then build the image using docker build -t test . then run the image using docker run -it --name test_container -v test-volume:/data test

--name : is the name of the container, -it : to run the shell mode -v : to mount the volume

You will find yourself inside the working directory /data

Now you can read and edit the test.txt as you want.

The file value can be accessible by all containers that use the test-volume .

Finally to delete the volume use docker volume rm VOLUME-NAME But make sure you stopped all containers that use the volume before deleting it.

For more details about volumes visit Use volumes.

Docker compose

All the containers will be defined in a single file called docker-compose.yml

Each container manages a particular component/service of your application and the docker-compose run them using only one command.

For the installation guide, I redirect you to the official website.

Then run docker-compose version to see your docker-compose version.

Now let’s take the Load balance exercise and try to automize it using docker-compose.

we need to have the same folders Web1, Web2 and myhaproxy .

Now create the docker-compose.yml file.

Then run docker-compose up -d

-d : for the detach mode.

To see your running containers use docker-compose ps

Now once your containers are running you can test by using curl 0 .

For more details about the docker-compose visit the docker-compose reference.

Conclusion

The purpose of this article was to introduce you to Docker and to help you better understand the solutions that can be brought to the various problems of developers.

The Startup

Get smarter at building your thing. Join The Startup’s +724K followers.

Sofiene Ben Khemis

Written by

Software Engineer. I’m not perfect. still running after my dreams. going to the moon 🌑

The Startup

Get smarter at building your thing. Follow to join The Startup’s +8 million monthly readers & +724K followers.

Sofiene Ben Khemis

Written by

Software Engineer. I’m not perfect. still running after my dreams. going to the moon 🌑

The Startup

Get smarter at building your thing. Follow to join The Startup’s +8 million monthly readers & +724K followers.

Medium is an open platform where 170 million readers come to find insightful and dynamic thinking. Here, expert and undiscovered voices alike dive into the heart of any topic and bring new ideas to the surface. Learn more

Follow the writers, publications, and topics that matter to you, and you’ll see them on your homepage and in your inbox. Explore

If you have a story to tell, knowledge to share, or a perspective to offer — welcome home. It’s easy and free to post your thinking on any topic. Write on Medium

Get the Medium app

A button that says 'Download on the App Store', and if clicked it will lead you to the iOS App store
A button that says 'Get it on, Google Play', and if clicked it will lead you to the Google Play store