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Sysadmin DeCal

Lab 9 - Containers (Docker)

Facilitator: lemurseven, kian

8 min read

Table of contents

  1. Overview
  2. Getting started with Docker
    1. Creating your first Docker container
    2. Running an interactive container
      1. Questions
    3. Dockerfiles
      1. Questions
    4. Dockerizing a Web Server
      1. Questions

Overview

This lab is designed to give you some hands-on experience with Docker! By the end of this assignment, you should be able to:

  • Create and use a Docker container interactively and create a Dockerfile, which allows you to declaratively define your containers.

Keep track of your answers to the questions, as you’ll need to submit them to Gradescope.

Getting started with Docker

You have a couple of options for installing Docker. It is recommended to follow the “Install using the apt repository” section.

After installing, run sudo usermod -aG docker $USER, then logout and login again. This adds your user to the docker group so you can run docker as a non-root user. This means you won’t have to type sudo docker all the time. This is optional but for the rest of the lab I’m going to assume that you did this.

Creating your first Docker container

To verify that you installed things correctly, try running:

docker run hello-world

You should see some friendly output like so:

Unable to find image 'hello-world:latest' locally
latest: Pulling from library/hello-world
1b930d010525: Pull complete
Digest: sha256:c3b4ada4687bbaa170745b3e4dd8ac3f194ca95b2d0518b417fb47e5879d9b5f
Status: Downloaded newer image for hello-world:latest

Hello from Docker!
This message shows that your installation appears to be working correctly.
...

This message shows that your installation appears to be working correctly. To generate this message, Docker took the following steps:

  1. The Docker client contacted the Docker daemon.
  2. The Docker daemon pulled the “hello-world” image from the Docker Hub.
  3. The Docker daemon created a new container from that image which runs the executable that produces the output you are currently reading.
  4. The Docker daemon streamed that output to the Docker client, which sent it to your terminal.

Some quick definitions from Docker’s website:

An image is a lightweight, stand-alone, executable package that includes everything needed to run a piece of software, including the code, a runtime, libraries, environment variables, and config files. Images are useful primarily for their speed, but images can also be used as a base to be built on top of in future images, as you’ll see later with Dockerfiles. In the last example hello-world was the image used to test our docker installation.

A container is a runtime instance of an image—what the image becomes in memory when actually executed. It runs completely isolated from the host environment by default, only accessing host files and ports if configured to do so. A container gets created upon executing docker run on an image.

Be sure to read through the output from running the hello-world image to get an understanding of what the Docker daemon was doing.

Running an interactive container

Now, let’s try to run a container interactively. This is useful if you ever need to play around and install stuff on a bare system without messing up your current system. Try running the following command:

docker run -it ubuntu:latest

The -i flag tells docker to keep STDIN open to your container, and the -t flag allocates a pseudo TTY for you. Basically you need both for you to have a shell into your newly started container. Try installing some packages from apt or just play around. It should look like a bare Linux system.

You can exit the container with CTRL+D.

Questions

  1. What user are you logged in as by default in the container?
  2. If you start and then exit an interactive container, and then use the docker run -it ubuntu:latest command again; is it the same container? How can you tell?

Dockerfiles

The natural question is, how are Docker images built? A Dockerfile is like the source code of an image. Rather, a Dockerfile allows you to define an image by specifying all of the commands you would type manually to create an image. Docker can then build images from a specified Dockerfile. These Dockerfiles can be put into version control and the images uploaded to online repositories. Can you see how this can be useful for deploying your application?

Dockerfiles are very powerful and have many different commands and features. We’ll go over a basic example, but you should check out the reference page if you are trying to do anything more complex.

Let’s jump in. We’re going to create an image that deploys your new startup’s app, Missile! Unfortunately, so far you only have the opening animation complete, and the source code is in b10/missile.py in repo - decal-labs.

Your program has a couple of dependencies. Namely, it requires Python and the python packages termcolor and pyfiglet to be installed. Here is a Dockerfile that puts those requirements into code, by installing Python 3 and the packages onto a base Fedora Linux image.

# Specify Fedora Linux as base image
FROM fedora:latest

# Install Python with yum (Fedora's Package Manager)
# Install required Python packages
RUN yum update -y && yum install -y python3 python3-pip && \
    python3 -m pip install pyfiglet termcolor

# Add the missile.py file to the final image
ADD missile.py /

# Specify the command to be run on container creation
CMD ["/usr/bin/python3", "missile.py"]

Note: there are some “best practices” for writing Dockerfiles that the above example doesn’t use, because it’s a basic example. If you’re interested in this stuff, check out this article.

Take a moment to appreciate how cool this is. We have a completely different Linux distribution with an application running on our system that can all be spun up with a single command. Now, when (if?) your startup finally takes off, scaling up will be a breeze!

Make sure you have both files named missile.py and Dockerfile respectively then build the image with the following command:

docker build -t missile:latest .

This tells Docker to look in the current directory for a Dockerfile to build, and builds it. The -t flag tells Docker to tag this build with the name missile:latest. Note that building the missile image will take a couple of minutes to complete.

You can see all of the images you’ve built on your machine with the docker images command.

Questions

  1. Run the image you just built with no flags. What do you observe?
  2. Write and build a Dockerfile based on ubuntu:bionic that installs the packages fortune and fortunes-min and runs the fortune executable (located in /usr/games/fortune after you install it). Note that you won’t need to use the -it flags when you run the container as fortune doesn’t need STDIN. Submit your Dockerfile with this lab. Hint: if you’re having trouble writing your Dockerfile, try booting an interactive container and installing both packages. How can you translate what you did interactively to a Dockerfile?
  3. Paste the output of running docker images command after completing questions 1 and 2.

Dockerizing a Web Server

For our last trick, we’re going to use Docker to run multiple Apache web servers inside containers.

For simplicity, you will not have to write this Dockerfile. Go ahead and pull the httpd image from Docker Hub. Now, it’s your job to figure out how to run three instances of the Apache containers on your machine.

Docker creates a separate network for containers, so you will need to forward your host port to your container’s port (this is called port forwarding, or port mapping). The container is listening on port 80 by default. It is your job to run each instance on ports 4000, 4001, and 4002. I recommend running the containers in detached mode with the -d flag. Detached mode will run a container in the background and print its new container ID. You can view running containers with docker ps.

Hints:

  • The -p flag takes in a colon separated pair of HOST_PORT:CONTAINER_PORT (it can actually accept a ton of more options, but don’t worry about that for now).
  • You can see if you were successful by executing curl localhost:4000 on your student VM. Check that you;’ve also done it correctly for ports 4001 and 4002.
  • Refer to the Docker commands slide if you’re stuck!

Questions

  1. While your three containerized Apache web servers are running in detached mode, paste the output of docker ps.
  2. Observe that in the output of docker ps, each container has an associated container ID. Explain why containers have IDs/Names rather than being named after the image, for example httpd.
  3. Now go ahead and stop your containers. Paste the command you used to stop one of the containers.

Congratulations! You’ve successfully Dockerized and ran a web server without affecting your setup on your machine :) There’s a lot more about Docker and containers to learn about, but I hope this was enough to wrap your head around the basic concepts and get some experience working with it.

For further reading - official documentation