Docker is an incredible tool that simplifies developement environemnt setup. It allows us to configure and save complex environments and their dependencies in a single image which we can execute at will. Nonetheless we have to configure our images and bundler is no exception.

Bundler works just fine inside a container. We can bind our project to the docker container and we can install our gems inside with a simple bundle install. We might do something like this for a jekyll blog:

version: '2'

services:
  jekyll:
    image: mycroft1891/jekyll-github
    volumes:
      - .:/site
    ports:
      - '80:4000'

This is my personal image which has ruby, node and yarn installed. The image will run bundle install if there is a gemfile in our project.

We can run docker-compose up and we will see that for the next 2-5 minutes bundler is installing our gems. Once bundler is done our blog is ready and we can create, edit and save our files without going through the whole process again but our gems are short lived. When we stop our container all the gems are lost. The next time we run docker-compose up we will see that bundler installs all our gems again.

When we run our container we want bundler to load any previously installed gems from disk and for that we have to use volumes. Docker allows us to create custom volumes and it creates volumes we specify in our docker-compose.yml by default. For our purpose we will create a volume to store our gems:

docker image create ruby_bundle

Now we have to bind it to the folder that bundler uses to store gems with our volume. Bundler installs all the gems in the $BUNDLER_PATH:

docker container run --rm -it ruby:2.5-alpine ash

echo $BUNDLE_PATH
# /usr/local/bundle

We can use that path to mount our volume and persist any downloaded gem for future use. For that we add our named volume to the docker-compose.yml file:

version: '2'

services:
  jekyll:
    image: mycroft1891/jekyll-github
    volumes:
      - ruby_bundle:/usr/local/bundle
      - .:/site
    ports:
      - '4000:4000'

volumes:
  ruby_bundle:
    external: true

Note that we are setting external to true here otherwhise docker would create a new volume using the folder and volume name resulting in a name like git-blog_ruby_bundle which wouldn’t allow us to reuse the volume for other containers.

No we can run docker-compose up again and we will see that bundler installs our gems again but if we stop the container and run docker-compose up again it loads all the gems from the volume and the startup time is much shorter.

We can also share this named volume with other containers as well.

Further reading:

• Docker Volumes
• Docker Containers