Restoring missing gems when running Jekyll in Docker

[mu88]

I'm trying to run my GitHub Page which is using Jekyll locally on my Windows machine (code of my page).

The Jekyll config _config.yml references the following plugins:

# all the other Jekyll settings

plugins:
  - jekyll-seo-tag
  - jekyll-sitemap
  - jemoji

I follow the official manual from here and created the following docker-compose.yml:

version: '2.2'

services:
  jekyll:
    image: jekyll/jekyll:3.8
    command: >
      sh -c "gem install jekyll-seo-tag jekyll-sitemap jemoji &&
             jekyll build &&
             jekyll serve --watch --force_polling --verbose"
    ports:
      - 4000:4000
    volumes:
      - .:/srv/jekyll

As you can see, at first I have to manually install all dependencies (which are specified in the plugins section of _config.yml).

When reducing the command to jekyll serve (as described in the docs), the following error occurs:

mu88githubio-jekyll-1  | ruby 2.6.3p62 (2019-04-16 revision 67580) [x86_64-linux-musl]
mu88githubio-jekyll-1  | Configuration file: /srv/jekyll/_config.yml
mu88githubio-jekyll-1  |   Dependency Error: Yikes! It looks like you don't have jekyll-seo-tag or one of its dependencies installed. In order to use Jekyll as currently configured, you'll need to install this gem. The full error message from Ruby is: 'cannot load such file -- jekyll-seo-tag' If you run into trouble, you can find helpful resources at https://jekyllrb.com/help/!
mu88githubio-jekyll-1  | jekyll 3.8.6 | Error:  jekyll-seo-tag
mu88githubio-jekyll-1 exited with code 1

The docs also mention the command bundle update, but this gives me:

mu88githubio-jekyll-1  | Could not locate Gemfile
mu88githubio-jekyll-1 exited with code 10

What bothers me with my current "solution" is that I have to specify the dependencies/plugins twice, both in docker-compose.yml and _config.yml.

Is it possible to get rid of this redundancy and make the Jekyll Docker image restore the necessary gems automatically when using jekyll serve and using _config.yml?

Thanks and have a great weekend!

PS: I've also placed this question on Stack Overflow last weekend, unfortunately no answers yet.

[pranabdas]

Could you try adding Gemfile in your project root with following content (also remove Gemfile.lock if exists):

source "https://rubygems.org"

gem "github-pages"

group :jekyll_plugins do
  gem "jekyll-sitemap"
  gem "jekyll-seo-tag"
  gem "jemoji"
end

Then you run this command once:

bundle config set --local path 'vendor' && bundle install

The gems will be installed in a folder named vendor locally. Remember to include it in .gitignore so that you don't include in your git history. If you delete this folder, you need to run above command again.

Now you should be able to serve your site locally by:

bundle exec jekyll serve

[mu88]

Great, that works, thank you 💪🏻

However, now the process logs that 92 gems were downloaded and installed. I'm wondering why this is necessary because with my previous approach, only the three missing gems were downloaded and installed - I assume the other 89 gems are already a part of the Docker image jekyll/jekyll.
Can this be optimized, i. e. download only the missing 3 gems and use the other ones from Docker?

[pranabdas]

Your project depends on more than the listed plugins (I got error while doing quick check). So, I used the github-pages gem, which includes all common (jekyll related) dependencies. In your case following Gemfile spec should be enough:

source "https://rubygems.org"

group :jekyll_plugins do
  gem "jekyll-paginate"
  gem "jekyll-sitemap"
  gem "jekyll-seo-tag"
  gem "jemoji"
end

Delete Gemfile.lock and vendor folder from your project if exists. Run bundle install followed by bundle exec jekyll serve.

[mu88]

Here is my docker-compose.yml:

version: '2.2'
services:
  jekyll:
    image: jekyll/jekyll:3.8
    command: >
      sh -c "bundle install &&
             bundle exec jekyll serve --watch --force_polling --verbose"
    ports:
      - 4000:4000
    volumes:
      - .:/srv/jekyll

And the Gemfile:

source "https://rubygems.org"

group :jekyll_plugins do
  gem "jekyll-paginate"
  gem "jekyll-sitemap"
  gem "jekyll-seo-tag"
  gem "jemoji"
end

But it still logs Bundle complete! 4 Gemfile dependencies, 42 gems now installed., whereas my previous version (with the duplicates) only downloads the four missing gems and logs 4 gems installed

[pranabdas]

I think, those are secondary dependencies. You can inspect Gemfile.lock to review what is being installed. Run bundle install && cat Gemfile.lock. bundle install without existing Gemfile.lock will attempt to install most updated versions possible. I think many dependencies in the jekyll image are outdated. Also I notice, if we don't specify jekyll version in the Gemfile, it's installing the latest version 4.x.x. If you like to stick to version 3.x.x, your Gemfile should be:

source "https://rubygems.org"

gem "jekyll", "~> 3.9"

group :jekyll_plugins do
  gem "jekyll-paginate"
  gem "jekyll-seo-tag"
  gem "jekyll-sitemap"
  gem "jemoji"
  gem "kramdown-parser-gfm"
end

However, it will still update the minor versions. This will happen every time you run docker. I would suggest, you commit the changes to jekyll image locally. In order to do that remove Gemfile.lock, and do the following:

docker run -ti -v ${PWD}:/srv/jekyll jekyll/jekyll:3.8 bundle install

Find the last running container id:

docker ps -l

Or if you are running multiple containers simultaneously, list them all and identify which is jekyll:

docker ps -a

Container id will look like: fc0e52914b4a. It's a SHA256 hash, but you don't need full hash. Unless there is conflict, first few characters are enough. Make commit with the same image name and tag:

docker commit fc0e52 jekyll/jekyll:3.8

Alternatively, you can apply new tag:

docker commit fc0e52 jekyll/jekyll:updated

Cleanup all stopped containers (optional):

docker container prune

Now, in your docker compose directly pass: bundle exec jekyll serve [OPTIONS], no need bundle install. If you changed image tag, change the image to jekyll/jekyll:updated. In case you remove your local image, you have to do the above again.