serra-vp/docker-1-mini

Project Summary

This is a brief introduction to Docker. Docker is a complex and useful tool, but we are going to use it in a very limited way. Its main use case for us, is to make the installation of other software seamless and idempotent (meaning the installation will not have any side-effects on our base system).

We will learn how to install docker, as well as a small number of commands that will be useful for us.

Installation

Ubuntu

1. The easiest way to install Docker on ubuntu is by using apt

   sudo apt install docker.io
   

2. Once you have docker installed there are a few other things you may want to configure to make using docker a little easier.

   • If you'd like to be able to run docker commands without having to use sudo run the following commands.

      sudo usermod -aG docker $USER
      newgrp docker
     

   • For more details see: https://docs.docker.com/install/linux/linux-postinstall/

3. To test your installation run

   docker run hello-world
   

Windows & MacOs

You can download docker for desktop here: https://www.docker.com/products/docker-desktop

What is Docker

For an in depth explanation you'll want the explore the documentation: https://docs.docker.com/get-started/

For our purposes you won't need to have a deep understanding of exactly what Docker is and can do, you'll just need to be able to run some simple commands to manipulate Docker containers.

So what is it? Docker is a tool that allows us as developers to containerize applications. In a way you can think of it as a very lightweight virtual machine, that in our case, is just going to be running a specific application. The advantage we are going to get by utilizing Docker is that we don't need to modify our base system to get the Dockerized applications to run correctly, the application is isolated in a Docker container, it makes configuration much easier for us, and eliminates many variables that can occur when trying to install software on many different machines (such as getting an entire class to install software in a certain configuration).

Key Concepts

• Images - These are pre-configured Docker containers, they can contain anything the image creator wants, but in many cases an image is designed to run a single application.

• Containers - You can think of these as currently running Docker images.

  • What do we mean by running? In Docker there is the concept of starting an image, an image that has been started is now running and could be considered a running Docker container.

For our use case we'll just be learning to fetch/run Images, stop running images, list the running Containers on the system.

Pull/Fetching Docker Images

1. Here we'll fetch a simple Docker image that is hosted on the docker hub: https://hub.docker.com

    docker pull macinv/flask-example
   

   You'll see the image being downloaded

2. Now that you've downloaded the image we can list all of the images that have been downloaded to our machine.

   docker images
   

Running an image

1. Let's run the image we just pulled down. The run command is what we want. There are a bunch of options available for this command referencing the documentation is a good idea.

   See: https://docs.docker.com/engine/reference/commandline/run/

   docker run -d -p 8080:8080 macinv/flask-example
   

   Command breakdown:

   • run the command to actually run the container
   • d runs the container in daemon mode
   • -p this option lets us bind ports from our host machine to the docker container. In this case we bind host (your machine) port 8080 to the container port 8080.
   • macinv/flask-example the name of the image we want to run

2. Try out that command

Active Docker Containers

1. You started a Docker container in the last step, let's see if we can check on that container

   docker ps
   

   This command will list all of the currently running containers on this host. Take a look at the metadata returned.

2. So, we know this container is running, but what is it doing? Remember that port that was bound to the container (8080)? Let's open that address in a web browser and see what happens.

   Link: http://localhost:8080

   You'll see that the Docker container is running a web server sends back a message

   Hello world! This is served by Flask in a Docker container

Stopping a Docker Container

1. Now that we've seen a running Docker container, how do we stop one? Simple, run the following command.

   docker stop [container name]
   

   Hint: unless explicitly set, container names will be random. You can set a container name in the run command For example: docker run --name=example_app -d -p 8080:8080 macinv/flask-example

Finished

We covered the most basic aspects of Docker, but there's no need to stop there if you're interested in learning more. There are more in-depth guides available, check out this link to the official documentation: https://docs.docker.com/