Docker Workshop (includes video)

ITEC 3870 Software Development II,
Ryan Cosentino

(License: CC BY-SA 4.0)

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Troubleshooting notes:

• Windows users may need to install the WSL2 update
• If Docker cannot turn on Hyper-V, you may need to enter your computer’s BIOS setup to turn it on
• Make sure you have enough space on your hard drive

What is Docker?

Docker is an OS-level virtualization software that is used to build and run isolated software environments called containers.

(Image Source)

In a way Docker is similar to virtual desktops in Windows or macOS where you can isolate open windows into different desktops. Although the windows in one desktop are isolated from those in another, they are still ordinary programs and not at all “virtualized” in the virtual machine sense. In this example different windows are isolated with help from the window manager, however with Docker processes are isolated at the kernel level. The specific technologies that make this possible will be summarized at the end.

Installing Docker

First download and install Docker Desktop.

In order to use the Docker command line client, Docker Desktop must be running. To verify that Docker is installed and running, run the following in your terminal:

$ docker version

The output should contain a section for

• Client: Docker Engine - Community, and
• Server: Docker Engine - Community

Images and Containers

Images are read only templates containing libraries, programs, data etc. that are used to create docker containers. A container is a runnable instance of an image. To use an OOP analogy, you can think of an image as a class and a container as an instance.

First Container

First we’ll create and run an Ubuntu container:

$ docker run -it --rm ubuntu bash

where

• -it is short for --interactive --tty which starts the container in interactive mode and lets you enter text via the terminal
• --rm deletes the container after it exits (but not the image)
• ubuntu is the name of the image
• bash is the command executed when the container starts
  (technically bash is already the default entrypoint for the Ubuntu image and is only used here to showcase the [COMMAND] argument)

First Container

If this is your first time using the Ubuntu image Docker will first download it from Docker Hub. If everything has gone according to plan, you should see a new prompt similar to

root@a5d9a7a5232c:/#

where

• root is the current user
• a5d9a7a5232c is the hostname (randomly generated by Docker)
• / is your current working directory (root directory in this case)
• # is another indication that you’re logged in as root (non root users have a $)

Note: In documentation, it is common to use a single dollar sign $ prompt (instead of something like root@a5d9a7a5232c:/#) for simplicity, especially when the information conveyed in a longer prompt isn’t relevant to the concepts at hand.

First Container

You can try running commands like ls to list files and folders:

$ ls
bin  boot  dev  etc  home  lib  ...  media  mnt  opt  proc  root  run  sbin  srv  sys  tmp  usr  var

or ps aux to list running processes:

$ ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  1.0  0.1   4112  3492 pts/0    Ss   17:13   0:00 /bin/bash
root        10  0.0  0.1   5900  3024 pts/0    R+   17:13   0:00 ps aux

Note: If this were a virtual machine, you would see tens, possibly hundreds of running processes instead of a small list of isolated ones.

First Container

You can also try factoring very large numbers:

$ factor 12345678901010101010101010987654321
12345678901010101010101010987654321: 19 853 32069 398053 465067 128312953569637

or:

$ yes Dr. Gunay is awesome
Dr. Gunay is awesome
Dr. Gunay is awesome
Dr. Gunay is awesome
Dr. Gunay is awesome
...

First Container

To quit, run the command exit. Since bash (PID 1) was the initial process used to start the container, once it exits, the container stops.

Because we used the --rm option, Docker should delete the container once it exits which you can verify by typing

$ docker container ls -a

however the Ubuntu image should still be saved locally:

$ docker image ls
REPOSITORY        TAG           IMAGE ID       CREATED       SIZE
ubuntu            latest        f63181f19b2f   12 days ago   72.9MB

Running Python

As another example, you can also try running Python instead of bash:

$ docker run -it --rm python:slim

Python 3.9.1 (default, Jan 12 2021, 16:56:42)
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.

>>> a = 5
>>> print(a + 2)
7

>>> exit()

Running Python 2.7

You can also create containers for different versions of Python:

$ docker run -it --rm python:2.7-slim

Python 2.7.18 (default, Jan 12 2021, 16:56:42)
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.

>>> a = 5
>>> print a+2
7

>>> exit()

Dockerfiles

Say you’d like to slightly customize an image available on Docker Hub. If you’re trying to run a Python application you could start off with the image python:slim, add your application and install any dependencies to create an image specific to your app. This is done using a Dockerfile.

Common Dockerfile Commands

• COPY <src> <dest>
  Copies files on the host at <src> to the container at the path <dest>

• RUN <command>
  Executes the command once inside the image

• ENV <key>=<value>
  Sets the environment variable <key> to <value>

• ENTRYPOINT <command>
  Executes <command> whenever the container starts

You can read more about all the different commands here.

Python Dockerfile

Create the file hello.py somewhere on your computer with the following lines:

import os

name = input('What is your name? ')
print(f'Hello {name}!')

print(os.environ.get('GOODBYE'))

Python Dockerfile

In the same folder as hello.py, create the file Dockerfile and add:

FROM python:3.9-slim

ENV GOODBYE=adios

# You can also add comments :0
COPY hello.py .

ENTRYPOINT python hello.py

$ docker build -t hello .

$ docker image ls
REPOSITORY        TAG           IMAGE ID       CREATED          SIZE
hello             latest        50b4f80bd32a   13 seconds ago   114MB

Running Your Image

Use the following command to run your image:

$ docker run -it --rm hello
What is your name? bob
Hello bob!
adios

After entering your name, the container should print a greeting, a closing, and then exit. Because of the --rm option, the container should be deleted.