Building a Docker Image for your Rails Application - Part 3

This is the last part of the series on building a Docker image for your Ruby on Rails application.

  • Part 1 is about making your application suitable to run inside a docker container.
  • Part 2 covers building a base image that contains the prerequisites needed for a typical Ruby on Rails app.
  • Part 3 (this post) - is about installing the application in the base image and running it as a container


In the previous posts, we have set up a base image with the environment to run a Rails app. Now we will start installing the application itself. The following steps will be necessary:

  • Install Ruby gems
  • Install javascript dependencies
  • Copy application files
  • Precompile the assets
  • Run the app

Install the Ruby gems:

The following lines in the Gemfile will do the trick:

COPY Gemfile .
COPY Gemfile.lock .
RUN bundle

You might be thinking: why not just copy all the application files and then run bundle install? Well, the docker build process executes a series of commands based on a context. The context, in this case, are the files that you include with the COPY command. If the context does not change between builds, docker will use a cached result instead of executing the command again. If we had copied the entire application in this step, whenever you changed any file, the build process would reinstall all the gems. Copying only the bundle-related files has the effect of invalidating the docker build cache only when the bundle changes.

Install javascript dependencies

For the same reason above, we just copy the yarn-related files before installing javascript dependencies:

COPY package.json .
COPY yarn.lock .
RUN yarn

Copy the application files:

The following commands will create a folder called app inside the image, and copy your application:

RUN mkdir -p /app
COPY . /app

There is one important thing to note here, tough. When docker runs the command COPY . /app, it will copy all files it finds in the application directory. If you are not careful, this might include log files (in case you are logging to the filesystem when developing), database files (if stored under /app), and other unnecessary stuff. Including unwanted files in your image can cause you two problems: it will increase the image size, and every time one of these file changes, it will trigger an unnecessary rebuild of the image

To solve this, you must have a .dockerignore file that tells docker which files you do not want in the resulting image. You can read about it here, if you are not familiar, or grab the example from our rails_docker_demo repository.

Precompile the assets

Next step, the application’s assets. Since we will be hosting the assets on the same container, we have to get them into the image:

RUN bundle exec rake assets:precompile DB_ADAPTER=nulldb NODE_ENV=development RAILS_ENV=staging SECRET_KEY_BASE=123

The DB_ADAPTER=nulldb variable is worth mentioning. When rails runs this rake task, it will boot the application in order to run the compiling. Depending on your environment, you might not have the environment varibles with database connection parameters at this moment, which would result in an error. So we use the nulldb adapter, database backend that translates database interactions into no-ops.

Finally, the EXPOSE command informs the TCP port the container will be responding on, and CMD provides a default command for running the server:

CMD ["bundle", "exec", "rails", "server", "-b", ""]

Build the image

Our Dockerfile is now ready. To build the image, execute the docker build command:

docker build -t coolrequest/rails_docker_demo .

This will execute the commands in the Dockerfile, using the current directory as context, save the image in your local Docker repository and tag it with the name coolrequest/rails_docker_demo

Run the app

Our application image is now ready. All you need to do to run it is provide the appropriate values for the environment variables. One simple way to do that is using a docker-compose file.

Suppose you don’t have postgres installed and want to run it using Docker as well. Create a docker-compose.yml file like this one:

version: '3'
    image: coolrequest/rails_docker_demo
      - 3000:3000
      - db
      - .env
    image: postgres
    restart: always
      POSTGRES_USER: rails_docker_demo
      POSTGRES_PASSWORD: my_pg_pass123
      - ./db/pgdata:/var/lib/postgresql/data
      - 5432:5432

… and run it using this command line:

docker compose up

That’s It

These are the steps necessary to make a Docker image from your Rails app. Using a similar approach, you could run it on a docker swarm or kubernetes cluster. Thanks for reading, and feel free to leave your thoughts on the comments section below.

Check out our repository with the full example here:


If you have any questions or feedback regarding this post, please leave your comment below. Keep in mind that comments are subject to moderation and will not be displayed immediately.

Sébastien - 31/01/2022 15:50

Great post thanks!

In my opinion the way you deal with dependencies can differ wether your are building an image for a local dev environment or for production. ` On my local dev machine I use bundle check || bundle cache in the entrypoint script. That way my rails dependencies are stored in vendor/cache in the rails project folder.

There is no need to “cache” the dependencies in an image used for a dev environment: This is only done for performance purpose on CI/CD or for production release.

Mauricio - 31/01/2022 17:05

That’s right, there are some minor differences in images used for development or production purposes. The Dockerfile described here is not optimized for production, and should work in both cases.

When I mentioned “cache” in the post, I was referring to the docker build cache (caching of image layers), not the bundler cache. The strategy of copying only Gemfile and Gemfile.lock before running bundler ensures that the resulting image layer containing the gems can be reused even if the rest of the application code changes.