Using Docker with Jekyll - Containers Used in Generating this Lesson

Overview

Teaching: 20 min
Exercises: 0 min

Questions

• What is an example of how I might use Docker instead of installing software?

• How can containers be useful to me for building websites?

Objectives

• Use an existing container image and Docker in place of complicated software installation work.

• Demonstrate how to construct a website using containers to transform a specification into a fully-presented website.

As previously mentioned earlier in the lesson, Docker can be helpful for using software that can be difficult to install. An example is the software that generates this lesson website. The website for this lesson is generated mechanically, based on a set of files that specify the configuration of the site, its presentation template, and the content to go on this page. When working on updates to this lesson, you might want to preview those changes using a local copy of the website. This requires installing Jekyll and depdencies such as Ruby and Gemfiles to your local computer which can be difficult to achieve given complexities such as needing to match specific versions of the software components. Instead you could use Docker and a pre-built Jekyll container image.

First we need to get a copy of the website to work with on your computer. In your shell window, in your docker-intro create a new directory build-website and cd into it. We will be expanding a ZIP file into this directory later.

Now open a web browser window and:

1. Navigate to the GitHub repository that contains the files for this session;
2. Click the green “Clone or download” button on the right-hand side of the page;
3. Click “Download ZIP”.
4. The downloaded ZIP file should contain one directory named docker-introduction-gh-pages.
5. Move the docker-introduction-gh-pages folder into the build-website folder you created above.

There are many ways to work with ZIP files

Note that the last two steps can be achieved using a Mac or Windows graphical user interface. There are also ways to effect expanding the ZIP archive on the command line, for example, on my Mac I can achieve the effect of those last two steps through running the command unzip ~/Downloads/docker-introduction-gh-pages.zip.

In your shell window, if you cd into the docker-introduction-gh-pages folder and list the files, you should see something similar to what I see:

$ cd docker-introduction-gh-pages
$ ls

AUTHORS			_episodes		code
CITATION		_episodes_rmd		data
CODE_OF_CONDUCT.md	_extras			fig
CONTRIBUTING.md		_includes		files
LICENSE.md		_layouts		index.md
Makefile		aio.md			reference.md
README.md		assets			setup.md
_config.yml		bin

You can now request that a container is created that will compile the files in this set into the lesson website, and will run a simple webserver to allow you to view your version of the website locally. Note that this command will be long and fiddly to type, so you probably want to copy-and-paste it into your shell window. This command will continue to (re-)generate and serve up your version of the lesson website, so you will not get your shell prompt back until you type control+c. This will stop the webserver, since it cleans away the container.

If you happen to have the make tool already installed…

We are taking an atypical approach in using the command that follows, since you are not required to have set up the make tool on your computer. However you may want to see whether you happen to have make installed anyway, by typing make docker-serve instead of the command below. At worst, this will fail and you can use the command shown below.

For macOS, Linux and PowerShell:

$ docker run --rm -it -v ${PWD}:/srv/jekyll -p 127.0.0.1:4000:4000 jekyll/jekyll:pages jekyll serve

For cmd.exe shells on Microsoft Windows:

> docker run --rm -it -v "%CD%":/srv/jekyll -p 127.0.0.1:4000:4000 jekyll/jekyll:pages jekyll serve

When I ran the macOS command, the output was as follows:

Unable to find image 'jekyll/jekyll:pages' locally
pages: Pulling from jekyll/jekyll
cbdbe7a5bc2a: Already exists 
aa8ae8202b42: Already exists 
b21786fe7c0d: Already exists 
68296e6645b2: Already exists 
6b1c37303e2d: Already exists 
4d49f4d60e44: Pull complete 
Digest: sha256:3741cb6d48b1ed3c544db4af9e2485fba31ddb5c2deb83a93b33fd252e8e2768
Status: Downloaded newer image for jekyll/jekyll:pages
ruby 2.7.1p83 (2020-03-31 revision a0c7c23c9c) [x86_64-linux-musl]
Configuration file: /srv/jekyll/_config.yml
            Source: /srv/jekyll
       Destination: /srv/jekyll/_site
 Incremental build: disabled. Enable with --incremental
      Generating... 
      Remote Theme: Using theme carpentries/carpentries-theme
                    done in 2.996 seconds.
 Auto-regeneration: enabled for '/srv/jekyll'
    Server address: http://0.0.0.0:4000
  Server running... press ctrl-c to stop.

In the preceding output, you see Docker downloading the image for Jekyll, which is a tool for building websites from specification files such as those used for this lesson. The line jekyll serve indicates a command that runs within the Docker container instance. The output below that is from the Jekyll tool itself, highlighting that the website has been built, and indicating that there is a server running.

Open a web browser window and visit the address http://localhost:4000/. You should see a site that looks very similar to that at https://carpentries-incubator.github.io/docker-introduction/.

Using a new shell window, or using your laptop’s GUI, locate the file index.md within the docker-introduction-gh-pages directory, and open it in your preferred editor program.

Near the top of this file you should see the description starting “This session aims to introduce the use of Docker containers with the goal of using them to effect reproducible computational environments.” Make a change to this message, and save the file.

If you reload your web browser, the change that you just made should be visible. This is because the Jekyll container saw that you changed the index.md file, and regenerated the website.

You can stop the Jekyll container by clicking in its terminal window and typing control+c.

You have now achieved using a reproducible computational environment to reproduce a lesson about reproducible computing environments.

Key Points

• You can use existing container images and Docker instead of installing additional software.

• The generation of this lesson website can be effected using a container.