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One Way To: Containerize a Ruby on Rails Application


While Ruby on Rails framework was not originally designed to be containerized, with it emerging before containerization becoming mainstream, with a little adjustment to its configuration and careful application architecture, it can be packaged up quite easily. Here one of the ways I have found to containerize a basic rails application for production (containerization for development and test to follow).

Why Containerize

There are many reasons for one to want to containerize a rails application, most of which tie into the benefits of containers. Below are a few of the pros and cons:


  • Scalability - Compared to traditional VM environments, containers are very quick to spin up and stop, allowing for fast on-demand scaling.
  • Portability - Applications in the container will run the same regardless of where they are deployed. This consistent reproducibility allows for easy deployment on a range of different machines.
  • Resource efficiency - Containerized applications reduce wasted resources. Each container only holds its application, while sharing the host's kernel. The containers can then be deployed efficiently with resource limits, to maximise the host instance's utilization.
  • Workflow - The whole workflow from test and development to deployment can be performed in a containerized environment. The environmental consistency across the workflow provides fast development setup on any machine, prevents cross-platform bugs, and identifies all dependencies.


  • Persistent storage - By design, local storage in containers is temporary, and is lost when the container is stopped. Coupled with the fact containers are ephemeral, persistent storage requires managing through carefully managed mounted volumes or backing services.
  • Performance - Not quite as fast as bare-metal due to overhead from various aspects of container architecture, such as overlay networking. However, performance is normally offset by resource efficiency.

Overall if you're building a stateless application with the need to scale efficiently and effectively, containerizing it could serve you well.



This post assumes you already have a simple Ruby on Rails application which you want to encapsulate in a container. I created a sample containerized Ruby on Rails application, which I will refer to throughout this post, the complete codebase can be found here: https://github.com/cpcwood/containerized_blog


Since containers are required to be ephemeral (short-lived), the application must be stateless and have no reliance on local storage for persistent or shared data. In Rails, this means designing the application to:

  • store client session data in cookies or a backing service
  • use backing services, such as external databases and AWS S3, for persistent data storage
  • logging to stdout and managing centralized logging externally

A common architectural starting point for a containerized application is the 12 factor app, which if followed will help cover many of the design quirks of applications running in containers.


As described in the 12 factor app, the application configuration is stored in environment variables. Therefore, go through the application and convert user configuration be extracted from the environment variables.

Update config/database.yml:

default: &default
  adapter: postgresql
  encoding: unicode
  host: <%= ENV.fetch("DB_HOST") { 'localhost' } %>
  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
  username: <%= ENV["DB_USERNAME"] %>
  password: <%= ENV["DB_PASSWORD"] %>
  port: 5432

  <<: *default
  database: <%= ENV.fetch("DB_NAME") { 'containerized_blog_development' } %>

  <<: *default
  database: <%= ENV.fetch("DB_NAME") { 'containerized_blog_test' } %>

  <<: *default
  database: <%= ENV.fetch("DB_NAME") { 'containerized_blog_production' } %>

Update config/storage.yml:

# ...
  service: S3
  access_key_id: <%= ENV.fetch('AWS_ACCESS_KEY_ID') { '' } %>
  secret_access_key: <%= ENV.fetch('AWS_SECRET_ACCESS_KEY') { '' } %>
  region: <%= ENV.fetch('AWS_REGION') { '' } %>
  bucket: <%= ENV.fetch('AWS_BUCKET') { '' } %>

Make sure to keep a note of the environment variables required to configure the application. To do this you can create a new directory and template config file, for example:

mkdir config/env_vars
vim config/env_vars/.env.template
# config/env_vars/.env.template

# Application Server Settings

# PSQL Database Credentials

# AWS Credentials

# Site Settings

These environment variables can then be added to the container environment during its creation by your container orchestrator.

Development and Test Environments

While the 12 factor app methodology disagrees with using config files for separate 'environments', such as development, test, Rails is designed around these environments and sometimes it is not practical to set the environment's environment variables each time you want to run. So, in this case, where we are just containerizing for production, it can be easier to use a gem such as dotenv to load environment files.

First, add the gem 'dotenv' to your Gemfile, under :development and :test, and install using bundle install.

Update your .gitignore file to ignore any env files, ensuring they are not checked into source control, possibly exposing secrets:

# …

Create the development and test .env files:

# config/env_vars/dev.env

# insert development config environment variables…
# config/env_vars/test.env

# insert test config environment variables…

Load the .env files using dotenv at the top of their respective environment files. For example:

# config/environments/development.rb
require 'dotenv'
# ...


Docker, is a set of platform as a service (PaaS) products used to define, build, and run Docker container images, and much more. It has quickly become the industry standard for creating containers and is very well documented, so well use it here.

Install Docker, using their installation guide for your machine.

Creating containers using Docker is done in three steps:

  1. Define the container image in a Dockerfile
  2. Build the container image
  3. Run the container image as a container

In this post we will perform the first two steps since once a container image is built, it can be published and be deployed on any machine with container runtime installed, such as Docker, CRI-O, and containerd.


A Dockerfile is a list of commands which when run will assemble a container image.

Create the .dockerignore

When copying files into the container image, use a .dockerignore file (similar to .gitignore file) to prevent unwanted files from copying and bloating the image size:

Create the .dockerignore file:

touch .dockeringore

Add patterns for files to be ignored, for example:

# OS X
Network Trash Folder
Temporary Items

# Rails

# others

Create the Dockerfile

Create a new file named Dockerfile in the root directory of the project:

touch Dockerfile

Builder Image

As each instruction in the Dockerfile adds a layer to the image, using a multi-stage build to compile client assets can be an easy way to minimize the end image size.

Image size optimization is important since a smaller image size improves the performance of the build and deployment as less data needs to be pulled to the running container cluster.

Pick a base image which is suitable for the project, ) and use it for the as the builder and main images. For example, ruby:2.7.2-alpine, official Ruby container images will already be optimised, and alpine is a very lightweight Linux distribution to keep image sizes down.

Then set the standard environment variables for the builder stage and create the working directory. Add temporary SECRET_KEY_BASE to allow for precompiling of assets. Using BUNDLE_PATH to keep the gem files local to the application allows for easier copying of the dependencies later:

# Dockerfile

# Builder Image
FROM ruby:2.7.2-alpine AS builder

ENV RAILS_ENV=production \
    NODE_ENV=production \
    APP_HOME=/opt/app \

ENV BUNDLE_PATH=$APP_HOME/vendor/bundle \

RUN mkdir -p $APP_HOME

Install application dependencies:

# Dockerfile
# …

RUN apk add --no-cache \
    build-base \
    postgresql-dev \
    nodejs \
    yarn \

Cache Gemfile & package.json

After the initial build of the container image, Docker will cache image layers, only rebuilding them when there has been a change. Since the application Gemfile and package.json do not change very often and add significant time to the build, it is common to copy these into the image early on, to allow the dependencies they install to be cached.

# Dockerfile
# …

COPY Gemfile* $APP_HOME/
RUN bundle config set without development:test:assets && \
    bundle install

COPY package.json yarn.lock $APP_HOME/
RUN yarn install --production=true 

Precompile Assets

Copy the source files and compile the assets, also remove any build artifacts and unrequired dependencies to prevent them from being copied into the main image later on:

# Dockerfile
# …


RUN bundle exec rails assets:precompile

RUN rm -rf $APP_HOME/node_modules && \
    rm -rf $APP_HOME/app/javascript/packs && \
    rm -rf $APP_HOME/log/* && \
    rm -rf $APP_HOME/spec && \
    rm -rf $APP_HOME/storage/* && \
    rm -rf $APP_HOME/tmp/* && \
    rm -rf $APP_HOME/vendor/bundle/ruby/2.7.0/cache/ && \
    find $APP_HOME/vendor/bundle/ruby/2.7.0/gems/ -name "*.c" -delete && \
    find $APP_HOME/vendor/bundle/ruby/2.7.0/gems/ -name "*.o" -delete 

Create the Main Image

Define the main image directly beneath the builder image in a similar fashion. Make sure to only include the application dependencies required for runtime.

# Dockerfile
# …

# Main Image
FROM ruby:2.7.2-alpine

ENV RAILS_ENV=production \
    NODE_ENV=production \

ENV BUNDLE_PATH=$APP_HOME/vendor/bundle \

RUN apk add --no-cache \
    imagemagick \
    postgresql-client \
    tzdata && \
    cp /usr/share/zoneinfo/Europe/London /etc/localtime && \
    echo "Europe/London" > /etc/timezone

RUN mkdir -p $APP_HOME

Add Docker User

Run application as unprivileged docker user:

# Dockerfile
# …

RUN addgroup -S docker && \
    adduser -S -G docker docker

USER docker

Copy Assets from Builder

Copy the compile assets and source code from the builder stage, making sure they are owned by the new docker user:

# Dockerfile
# …

COPY --chown=docker:docker --from=builder $APP_HOME $APP_HOME

Expose Port

Expose the port the application server will be running on to allow external services and requests to contact the server:

# Dockerfile
# …


Startup Script

Since the container could be created at the beginning of the applications life, or midway through, the database might not be set up or fully migrated to the current image's state.

Create a startup script to first create or migrate the database and then start the application server.

Firstly, create a new rake task to check if the application database exists:

# lib/tasks/db_exists.rake

namespace :db do
  desc 'Checks to see if the database exists'
  task exists: :environment do
  rescue StandardError
    exit 1
    exit 0

Then create the startup shell script:

# scripts/container-startup.sh

bundle exec rake db:exists && bundle exec rake db:migrate || bundle exec rake db:setup
bundle exec rails server -b -p 5000

Finally, run the startup script upon container creation:

# Dockerfile
# …

CMD ["./scripts/container-startup.sh"]

Take a look at the full Dockerfile defined above here.

Build the Container Image

Build the container image from the Dockerfile is relatively simple using Docker. Simply run:

sudo docker build -t <container-name> .

Once the container image has been built, it is ready to be published to DockerHub and or deployed.

Test Using Docker Compose

While docker can be used to run container images directly, it is not particularly user friendly and repeatable when attempting to run multiple containers, such as the application and database. Docker compose can be used to configure and run multi-container applications in a repeatable manner. Therefore, it is useful to test your production container images locally, in a pseudo-staging environment.

Define the Application

Create a docker-compose.yaml in the root directory of the project defining the application and database services. For example:

version: "3.7"

    image: <container-name>
      - 'config/env_vars/.env'
      - postgres
      - '5000:5000'

    image: postgres:13
      POSTGRES_USER: cpcwood
      POSTGRES_PASSWORD: password
      PGDATA: /var/lib/postgresql/data/pgdata
      - '5432:5432'
      - /var/db/psql/13:/var/lib/postgresql/data
      test: pg_isready -U cpcwood -h
      interval: 5s

The above docker-compose file sets up two services, app and postgres, which run the Ruby on Rails application container and a PostgreSQL database container respectively.

The app service:

  • sets the container image as the name of the container image built earlier
  • loads environment variables from the config/env_vars/.env file
  • won't start until the postgres service is started
  • exposes port 5000 to port 5000 of the local machine

The postgres service:

  • uses the official postgres image, which will be pulled from dockerhub if not available locally
  • sets the user, password, and data location
  • exposes port 5432 to port 5432 of the local machine
  • mounts a local volume into the image data location directory, this allows for data to persist between container mounts (ensure the mounted directory /var/db/psql/13 is the correct directory on the local machine for your postgres data/)
  • performs a health check to ensure the container stay live

Create Environment File

When deploying with a container orchestrator, such as Kubernetes, the config will likely be created in its own object. However, for running the container locally using docker-compose it can be convenient to store the environment variables in a file and load them into the container on creation (make sure not to check it into your source control).

Create the environment file which is to be read and exported into the app container on creation. For example, create config/env_vars/.env from the template .env.template file created earlier.

Note: the database will not be available on localhost, instead it will be available on the name of the database service: postgres. So, in the example case, set DB_HOST=postgres

Start the Application

Use the docker-compose CLI to start the application:

sudo docker-compose up

Once started the application should now be live on port 5000 on your local machine. Test it by visiting in your browser.

What's next


Deploy the application using a container orchestrator, such as Kubernetes or Docker Swarm. Alternatively, use a managed system such as AWS's Amazon ECS or Google's GKE.


Building, publishing, and deploying containers can be performed during a continuous integration and continuous delivery (CI/CD) workflow, using services such as CircleCI or self-hosted Jenkins.

With further configuration, CI/CD services can also provide the ability to copy pre-compiled assets to CDN instead of using the backend application server to serve them, lightening up the image size and load, while speeding up response times for clients.

Development & Test Environments

Setup containerized development and test environments to allow for a consistent environment throughout the application's workflow, and quick setup on any machine with Docker installed.

Explore the Container

You can execute commands in running containers either directly or by running an interactive shell:

  • spin up the container using sudo docker-compose up
  • get the id of the running container using sudo docker containers ls and reading the CONTAINER ID
  • open shell in container by running the shell executable command in the container: ```sudo docker exec -it /bin/sh
  • explore

Optimize Container Image Size

There is almost always room for further container image size optimization. Try analysing and reducing the image size using tools like dive.

Lockdown Container Security

Since containers are just processes running on a host machine with some special configuration to provide isolation, careful consideration must be given to security during the application architecture and container definition.

Some good places to start are:


Drop me a message through the contact section of this site or via LinkedIn if you have any comments, suggested changes, or see any bugs.