Introduction to Docker Containers

Setup

Older version of Docker Desktop

If you had previously installed Docker Desktop on your system, you need to make sure that what you have installed is up to date. The latest Docker Desktop and the accompanying Docker Engine contain many useful tools to help administrating your images and containers.

This material is written on: - Docker Dekstop version 4.35.1 (173168) - Docker Engine: 27.3.1 - Docker Compose: v2.29.7-desktop.1

It is possible that by the time that you read this setup, the versions you have will be higher. That would be a good thing!

Links to download and install Docker Desktop

• Select the appropriate link for your computing platform:
  • Mac
  • Windows
    • You should run Linux containers when installing Docker Desktop for Windows
    • It is recommended that you use WSL2 option when installing Docker Desktop on Windows
  • Linux

• Successful installation and startup of Docker will show the Docker Desktop Dashboard
  • This screenshot is taken on a Mac, but the GUI should be the same across platforms

• You should also carry out a test for Docker inside a terminal.
  • Mac: If you have not used a Terminal on Mac before, click the Magnifying Glass icon (top right) and type in Terminal to find the app.
  • Windows: It is highly recommended that you download and use the Terminal App from Windows Store. For Windows 11+, this has become a default application.

Docker Hub

• Docker Hub is one of the public repository for Docker images (think GitHub for container images).
• You should register for Docker Hub account at https://hub.docker.com and use it to log into your Docker Desktop environment (similar to how you link your GitHub account to GitHub Desktop, if you use GitHub Desktop).

Introduction to Docker

Motivation

• Limit the chaos between computing platforms of
  • Development and production
  • Developers, Testers, and Users
  • Dependencies of various software components across a full-stack infrastructure
  • …
• Inspiration
  • Shipping containers
  • Ship anything under any forms anywhere

Getting started

• Launch your Docker Desktop app
• Open the built-in terminal and run

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docker version
 

Overview of Docker

• Docker is client-server application.
  • Docker daemon (Engine): receives and processes incoming Docker API request and requires root privilege.
  • Docker Hub registry: collection of public images (https://hub.docker.com/).
  • Docker client : Talks to the Docker daemon via the docker API and the registry API.

example Hands-on: Hello world

• Docker containers are instantiated from Docker images.
• You can check availability of local images and containers.

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docker image ls
docker container ls
 

• If you click on the Images and Container tabs, you can also confirm that there is no exissting container or image on your Docker envrironment.

• We want to issue the following to start a service that will echo hello world to the screen.
  • This requires a Linux container to run the echo command.

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docker run alpine echo hello, world
docker image ls
docker container ls 
docker container ls --all
 

• The outcomes of the above commands are shown in the screenshot below
  • User want to run a Linux command using an alpine Linux container
  • There is no locally available alpine image, therefore Docker goes ahead and download the image.
  • Docker uses the image as a blueprint to launch a container, which then executes the echo hello, world command.
  • The container is shutdown after the execution.

• docker run alpine echo hello world
  • docker: invoke the container engine.
  • run: subcommand to run a container.
  • alpine: name of the image based on which a container will be launched.
  • echo hello, world: the command to be executed in the container environment.
• docker image ls
  • Now an image shows up (it has just been downloaded)
  • The Image tab should also show the alpine image
• docker container ls
  • There is no running container listed. Although, you can see from the GUI that there was a container there.
• docker container ls --all
  • This command shows all containers, including those that finished running.

example Hands-on: Interactive container

• We can launch a container and get into the shell of the container.

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docker run -it ubuntu bash
 

• You are now in a new prompt: a shell inside the container
  • notice the changes in the prompt comparing to the previous prompt where you type in the docker run … command
  • This is important when you are on the terminal to know whether you are inside the container’s environment or the host machine’s environment.
• -it: combination of -i and -t.
  • -i tells Docker to connect to the container’s stdin for interactive mode
  • -t tells Docker that we want a pseudo-terminal
• Let’s attempt to run figlet inside the container (# prompt)

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figlet hello
 

• There will be an error: bash: figlet: command not found
• The current container does not have the figlet program yet.
• Let’s install figlet inside the container (# prompt)

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apt update -qq
apt install -y -qq figlet
figlet hello
 

question Exercise: exit container

• Type exit to shutdown the container and back to your normal terminal.
• Repeat the process of launching an interactive container from start and try running figlet again.
• Is the figlet program still there?

Solution

• The figlet program is no longer available again.
• Notice the recenlty shutdown container and the newly created container from the upper half of the GUI.

example Hands-on: Background container

• You should have already exited out of the container shell and back to the host machine environment.
• Run the following command
  • Press Ctrl-C to stop after a few time stamps.

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docker run ubuntu /bin/sh -c "while date; do sleep 1; done"
 

• Run the following command

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docker run -d jpetazzo/clock
docker container ls
 

• Notice in the screenshot that you are now returned to the host machine’s terminal.
• A running container is shown in both the docker container ls command and in the GUI’s container tab.
• The hash string, underlined by the red line, is a unique identifier for containers.

• Use the first four characters of your container ID to view the log of the running Docker container
  • From the above screenshot, it is cd16
• Use --tail N to only look at latest N lines of the log.

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docker logs --tail 5 cd16
 

• To kill a running container, you can use docker kill with the container ID or click on the Stop or Trash Can icons on the container’s line inside the GUI’s Container tab.

Docker Images

Docker images

• Image = files + metadata
• The files form the root filesystem of the container
• The metadata describes things such as:
  • The author of the image
  • The command to execute in container when starting it
  • Environment variables to be set
  • …
• Images are made of layers, conceptually stacked on top of each other.
• Each layer can add, change, and remove files and/or metadata.
• Images can share layers to optimize disk usage, transfer times, and memory use.

Example of a Java webapp

• CentOS base layer
• Packages and configuration files added by our local IT
• JRE
• Tomcat
• Our application’s dependencies
• Our application code and assets
• Our application configuration

The read-write layer

Containers versus images

• An image is a read-only filesystem.
• A container is an encapsulated set of processes running in a read-write copy of that filesystem.
• To optimize container boot time, copy-on-write is used instead of regular copy.
• docker run starts a container from a given image.

• Object-oriented analogy
  • Images are conceptually similar to classes
  • Layers are conceptually similar to inheritance
  • Containers are conceptually similar to instances

How do we change an image?

• It is read-only, we don’t.
• We create a new container from the image
• We make changes to the container.
• When we are satisfied with the changes, we transform them into a new layer.
• A new image is created by stacking the new layer on top of the old image.

Image namespaces

• Official images (ubuntu, busybox, …)
  • Root namespace.
  • Small, distro images to be used as bases for the building process.
  • Ready-to-use components and services (redis, postgresl …)
• User (and organizations) images: <registry_name>/<image_name>:[version]
  • linhbngo/csc331:latest
• Self-hosted images
  • Images hosted by third party registry
  • URL/<image_name>

example Hands-on: listing local images and searching for new images

• At this point, you should have at least two images downloaded to Docker’s local repository: alpine and ubuntu.

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docker image ls
 

• The command should show two outputs clearly.
• You should also see the images listed in the Image tab of Docker Desktop, on the top portion of the GUI.

• We can search for available images in the public Docker Hub

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docker search mysql
 

General steps to create an image

• Create a container using an appropriate base distro
• Inside the container, install and setup the necessary software
• Review the changes in the container
• Turn the container into a new image
• Tag the image

example Hands-on: create a container with a base distro

• Launch an interactive bash shell on a running container

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clear
docker run -it ubuntu /bin/sh
 

• Install figlet, test, then exit the container
  • Remember the hash id of your container

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apt update -qq
apt install -y -qq figlet
figlet hellp
exit
 

• From the screenshot above, next to the top container (the one that just got started and stopped 56 seconds ago), you can see the ID cb2149...
• It is possible to check for the differences between this container and the base image with the ID and docker diff

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docker diff cb21
 

• A: A file or directory was added
• C: A file or directory was changed
• D: A file or directory was deleted

example Hands-on: commit changes into a new image

• We are still using the container from the previous example.
  • From the first screenshot in the previous example, next to the top container (the one that just got started and stopped 56 seconds ago), you can see the ID cb2149...
• Run the following commands

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docker commit cb21 linhbngo/ubuntu_figlet:1.0
docker image ls
 

• The docker commit ... command created a new image named ubuntu_figlet that is associated with the DockerHub account linhbngo (my DockerHub account). This combination (linhbngo/ubuntu_figlet) is called a repository, and has the tag 1.0.
• The docker image ls command shows this image and other images available locally for Docker.

• In reviewing the previous screenshot, you can se that a new image has been created with a size of 144MB.
  • The original Ubuntu image has a size of 101MB.
• We can examine the layers of the newly committed image by running the command docker history ... as shown below.

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docker history 2379
 

• The previous screenshot also shows that the new layer making up the new image has a size of 43.3MB. This, adding to the base ubuntu image, is what giving the new image the size of 144MB (101 + 43.3).
  • Docker will not duplicate the base layer, but will reuse this layer storage when additional new images are created from the same base.
• You can now push the new image up to Docker Hub:
  • If you are not logging into Docker Hub from DockerDesktop, or are running on an external terminal, you will need to run docker login first to sign in to your Docker Hub account.

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docker push linhbngo/ubuntu_figlet:1.0
 

Automatic image construction: Dockerfile

• A build recipe for a container image.
• Contains a series of instructions telling Docker/Podman how an image is to be constructed.
• The docker build command builds an image from a Dockerfile.

example Hands on: writing the first Dockerfile

• The following commands are done in the terminal (Ubuntu WSL on Windows/Mac Terminal).

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cd
mkdir myimage
cd myimage
nano Dockerfile
 

• Type the following contents into the nano editor.
  • To save and quit nano, press Ctrl-X (or Control-X for Mac)
  • Type Y when asked to Save modified buffer
  • Press Enter.

• Content of the above Dockerfile
  • FROM: the base image for the build
  • RUN: represents one layer of execution.
  • RUN commands must be non-interactive.
• To build the image, you can run the following commands:
  • Assumption: you are still inside myimage
  • This could be checked with pwd.

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pwd
ls
docker build -t linhbngo/ubuntu_figlet:2.0 .
 

• -t indicates a tag named figlet will be applied to the image.
• . indicates that the Dockerfile file is in the current directory.

• Running docker image ls and examining the GUI’s Image tab, you will see that there are now two tags for the same repository name. If you run docker push and check your Docker Hub repo, you will see the second tag now stored in the same repository

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docker push linhbngo/ubuntu_figlet:2.0
 

question Exercise: test new image

• Test run the new ubuntu_figlet image by launching an interactive container using this image, then immediately run figlet hello world.

Solution

• Replace linhbngo with your own DockerHub account

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docker run -it linhbngo/ubuntu_figlet:2.0 /bin/bash
figlet hello world
exit
 

Infrastructure as Code

Overview

• Containers can be customized to run as programs/services
  • Barebone containers with just enough dependencies packages
• Customizable parameteres to feed to containers at start time
• Ephemeral/persistent storages
• Network infrastructures

CMD and ENTRYPOINT

• Both commands enable containers to run a default program or script.
• CMD executes a default program or script with a predefined set of parameters.
  • To be run if the container is invoked without any command.
• ENTRYPOINT defines a default program or script with a predefined set of parameters, but also allows users to append additional parameters to docker run call.

example Hands on: CMD

• Edit your Dockerfile so that it has the following content

• Rebuild the image with the tag linhbngo/ubuntu_figlet:3.0.
• Run the following command

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cd
cd myimage
docker build -t linhbngo/ubuntu_figlet:3.0 .
docker run -it linhbngo/ubuntu_figlet:3.0
 

question Exercise: storage consumption

• Run the following commands

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docker image ls
 

• Did we use any additional storage for this new image?
  • Hint: Try running docker system df

example Hands on: ENTRYPOINT

• Edit Dockerfile as follows:

• Rebuild the image with the tag linhbngo/ubuntu_figlet:4.0.
• Run the followings:

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docker build -t linhbngo/ubuntu_figlet:4.0 .
docker run linhbngo/ubuntu_figlet:4.0
docker run linhbngo/ubuntu_figlet:4.0 golden rams
 

• Notice that the first docker run, without any input parameters, does not generate any text.
• The second docker run takes golden rams and feeds it to the figlet command specified by ENTRYPOINT.

example Hands on: Using both ENTRYPOINT and CMD

• ENTRYPOINT and CMD can be used together.
• The command line arguments are appended to those parameters.
• Edit Dockerfile as follows:

• Rebuild the image with the tag linhbngo/ubuntu_figlet:5.0.
• Run the followings:

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docker build -t linhbngo/ubuntu_figlet:5.0 .
docker run linhbngo/ubuntu_figlet:5.0
docker run linhbngo/ubuntu_figlet:5.0 golden rams
 

• Caveat with ENTRYPOINT: /bin/bash does not work as expected.
  • Need to override with --entrypoint flag.

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docker run -it linhbngo/ubuntu_figlet:5.0
docker run -it --entrypoint bash linhbngo/ubuntu_figlet:5.0
exit
 

Infrastructure as Code: Storage

Overview

• Docker images are immutable (read-only)
• Docker containers are mutable, but are available only for the running duration of the containers.
  • Once containers are shutdown, to retain modifications, they need to be made immutable.
• How do we reconcile new data into Docker images/containers?
  • Copy data into images during the building process
  • Mount storage directory from host machines into containers at run time

COPY

• COPY src dst
• Copy contents into the image prior to the installation steps.
• Need to be done prior to RUN statements.

example Hands on: Importing and building external code using COPY

• Inside the myimage directory
• Create the following file called hello.c:

• Create the following Dockerfile called Dockerfile.hello:

• You can build an image with a specific Dockerfile

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docker build -t linhbngo/hello:1.0 -f Dockerfile.hello .
docker run linhbngo/hello:1.0
 

example Hands on: Mounting external storage to running containers

• Create a directory called src inside myimage.
• Copy hello.c into this directory.

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mkdir src
cp hello.c src/
 

• Create the following Dockerfile called Dockerfile.gcc:

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docker build -t linhbngo/gcc:1.0 -f Dockerfile.gcc .
docker run -it -v ./src:/ext_src linhbngo/gcc:1.0
 

• From the screenshot below, notice that after exiting out of the container, the newly created binary file hello still persists.

Infrastructure as Code: Networking

Networking for container

• How can services provided by a container become available to the world?
  • Piggyback on the host machine’s network.
  • Docker has supporting drivers for containers:
    • bridge: The default network driver.
    • host: Remove network isolation between the container and the Docker host.
    • none: Completely isolate a container from the host and other containers.
    • overlay: Overlay networks connect multiple Docker daemons together.
    • ipvlan: IPvlan networks provide full control over both IPv4 and IPv6 addressing.
    • macvlan: Assign a MAC address to a container.
• Why do we have to map ports?
  • By default, containers are not exposed to the world.
  • Internally to host, containers have their own private addresses
    • Services have to be exposed port by port.
    • These have to be mapped to avoid conflicts.

example Hands on: A simple web server

• Run the following commands inside the Docker terminal

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docker run -d -P nginx
docker container ls
 

• -P: make this service reachable from other computers (--publish-all)
• -d : run in background
• Where is the port?

• Run the following command

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docker inspect --format '' nginx 
 

• The outconme of the above command is {"80/tcp":{}}.
• In other words, the container exposes its own internal port, 80. In turn, this port is mapped to the external port 55000, as shown in the screenshot.
• You can view the nginx webserver by visiting 127.0.0.1:55000

example Hands on: manual allocation of port numbers

• In the previous example, the port randomly assigned by the host machine was 55000.
• This could be specified via the -p flag as follows:

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docker run -d -p 8000:80 nginx
docker run -d -p 8080:80 -p 8888:80 nginx
 

• Convention: port-on-host:port-on-container
• Check out the web servers at all of these ports