Docker deployment in Ruby on Rails - Mini Project: Build & Apply

Docker Deployment for a Rails Application

📖 Scenario: You have built a simple Rails web application and want to deploy it using Docker. Docker helps package your app and its environment so it runs the same everywhere.

🎯 Goal: Build a Docker setup for your Rails app by creating a Dockerfile, adding configuration for the database, and writing a Docker Compose file to run the app and database together.

📋 What You'll Learn

Create a Dockerfile with Ruby and Rails setup

Add a configuration variable for the Rails environment

Write a Docker Compose file to run the Rails app and PostgreSQL database

Ensure the app container depends on the database container

💡 Why This Matters

🌍 Real World

Docker is widely used to package and deploy Rails applications consistently across different machines and cloud platforms.

💼 Career

Knowing how to containerize Rails apps with Docker and Docker Compose is a valuable skill for backend developers and DevOps engineers working on modern web applications.

1

Create the Dockerfile for the Rails app

Create a file named Dockerfile with these exact lines to set up Ruby 3.2, install dependencies, copy the app code, and set the default command to start the Rails server.

Ruby on Rails

# Your code here

Hint

Start from the official Ruby image, install Node.js and PostgreSQL client, set working directory, copy Gemfile files, run bundle install, then copy the rest of the app and set the command to start Rails server.

2

Add environment variable for Rails environment

In the Dockerfile, add a line to set the environment variable RAILS_ENV to development right after setting the working directory.

Ruby on Rails

FROM ruby:3.2

RUN apt-get update -qq && apt-get install -y nodejs postgresql-client

WORKDIR /myapp
# Add environment variable RAILS_ENV=development below
# Your code here

COPY Gemfile /myapp/Gemfile
COPY Gemfile.lock /myapp/Gemfile.lock

RUN bundle install

COPY . /myapp

CMD ["rails", "server", "-b", "0.0.0.0"]

Hint

Use the ENV instruction in Dockerfile to set RAILS_ENV to development.

3

Create a Docker Compose file to run app and database

Create a file named docker-compose.yml with a version of "3.8". Define two services: db using the postgres:15 image with environment variables POSTGRES_USER: postgres and POSTGRES_PASSWORD: password, and web using the build context . and ports mapping 3000:3000. Make sure web depends on db.

Ruby on Rails

FROM ruby:3.2

RUN apt-get update -qq && apt-get install -y nodejs postgresql-client

WORKDIR /myapp

ENV RAILS_ENV=development

COPY Gemfile /myapp/Gemfile
COPY Gemfile.lock /myapp/Gemfile.lock

RUN bundle install

COPY . /myapp

CMD ["rails", "server", "-b", "0.0.0.0"]

# Create docker-compose.yml file with db and web services
# Your code here

Hint

Use version: "3.8" at the top. Define db service with postgres image and environment variables. Define web service with build context, ports, and depends_on db.

4

Add command to wait for database before starting Rails server

In the docker-compose.yml file, under the web service, add a command that uses sh -c to wait for the database on db:5432 using until and then starts the Rails server with rails server -b 0.0.0.0.

Ruby on Rails

FROM ruby:3.2

RUN apt-get update -qq && apt-get install -y nodejs postgresql-client

WORKDIR /myapp

ENV RAILS_ENV=development

COPY Gemfile /myapp/Gemfile
COPY Gemfile.lock /myapp/Gemfile.lock

RUN bundle install

COPY . /myapp

CMD ["rails", "server", "-b", "0.0.0.0"]

# docker-compose.yml
version: "3.8"
services:
  db:
    image: postgres:15
    environment:
      POSTGRES_USER: postgres
      POSTGRES_PASSWORD: password
  web:
    build: .
    ports:
      - "3000:3000"
    depends_on:
      - db
    # Add command to wait for db and start rails server below
    # Your code here

Hint

Use command: sh -c "until nc -z db 5432; do echo 'Waiting for db'; sleep 1; done; rails server -b 0.0.0.0" to wait for the database before starting the server.

Practice

1. What is the main purpose of using Docker for deploying a Rails application?

easy

A. To package the app and its environment together for consistent deployment

B. To write Rails code faster

C. To replace the Rails framework

D. To automatically fix bugs in the app

Solution

1. Step 1: Understand Docker's role

   Docker packages the app with all dependencies so it runs the same everywhere.

2. Step 2: Compare options

   Options B, C, and D describe unrelated or incorrect uses of Docker.

3. Final Answer:

   To package the app and its environment together for consistent deployment -> Option A

4. Quick Check:

   Docker deployment = packaging app + environment [OK]

Hint: Docker bundles app + environment for consistent runs [OK]

Common Mistakes:

• Thinking Docker speeds up coding
• Confusing Docker with Rails framework
• Believing Docker fixes app bugs automatically

2. Which line correctly exposes port 3000 in a Dockerfile for a Rails app?

easy

A. EXPOSE port 3000

B. EXPOSE '3000'

C. EXPOSE:3000

D. EXPOSE 3000

Solution

1. Step 1: Recall Dockerfile syntax for exposing ports

   The correct syntax is simply EXPOSE followed by the port number without quotes or extra words.

2. Step 2: Evaluate options

   EXPOSE 3000 matches the correct syntax. Options A, B, and D have invalid syntax.

3. Final Answer:

   EXPOSE 3000 -> Option D

4. Quick Check:

   Expose port syntax = EXPOSE 3000 [OK]

Hint: Use EXPOSE followed by port number without quotes [OK]

Common Mistakes:

• Adding quotes around port number
• Writing extra words like 'port'
• Using colon instead of space

3. Given this Dockerfile snippet for a Rails app:

FROM ruby:3.1
WORKDIR /app
COPY . .
RUN bundle install
EXPOSE 3000
CMD ["rails", "server", "-b", "0.0.0.0"]

What happens when you run this container and visit http://localhost:3000 on your host machine?

medium

A. You can access the Rails app running inside the container

B. The app will not be accessible because the port is not exposed

C. The app will crash because of missing files

D. The server will bind only to localhost inside the container

Solution

1. Step 1: Analyze Dockerfile commands

   The Dockerfile exposes port 3000 and runs Rails server bound to 0.0.0.0, allowing external access.

2. Step 2: Understand port exposure and binding

   Exposing port 3000 and binding to 0.0.0.0 lets the host access the app via localhost:3000.

3. Final Answer:

   You can access the Rails app running inside the container -> Option A

4. Quick Check:

   Expose + bind 0.0.0.0 = accessible app [OK]

Hint: Bind Rails to 0.0.0.0 and expose port to access app [OK]

Common Mistakes:

• Thinking EXPOSE alone publishes port to host
• Binding server only to localhost inside container
• Assuming missing files cause crash without evidence

4. You wrote this Dockerfile for your Rails app:

FROM ruby:3.1
WORKDIR /app
COPY . .
RUN bundle install
EXPOSE 3000
CMD ["rails", "server"]

Why might your app not be accessible from your host machine at http://localhost:3000?

medium

A. Because the port 3000 is not exposed in the Dockerfile

B. Because the Rails server is not bound to 0.0.0.0, so it listens only inside the container

C. Because the WORKDIR is incorrect

D. Because bundle install was not run

Solution

1. Step 1: Check server binding in CMD

   The CMD runs 'rails server' without '-b 0.0.0.0', so it binds to localhost inside container only.

2. Step 2: Understand port exposure and binding

   Even though port 3000 is exposed, binding only to localhost prevents external access from host machine.

3. Final Answer:

   Because the Rails server is not bound to 0.0.0.0, so it listens only inside the container -> Option B

4. Quick Check:

   Missing bind 0.0.0.0 = no host access [OK]

Hint: Always bind Rails server to 0.0.0.0 for external access [OK]

Common Mistakes:

• Assuming EXPOSE publishes port to host automatically
• Thinking WORKDIR affects network access
• Believing bundle install affects accessibility

5. You want to deploy a Rails app with Docker and ensure the app reloads code changes during development inside the container. Which Dockerfile and command setup is best?

hard

A. Copy code once in Dockerfile and run: rails server without binding

B. Expose port 3000 but do not bind Rails server to 0.0.0.0

C. Use a volume to mount your app code and run: rails server -b 0.0.0.0

D. Run Rails server with rails server -b localhost and no volume

Solution

1. Step 1: Understand code reload needs

   For code changes to reflect immediately, the app code must be mounted as a volume, not copied once.

2. Step 2: Ensure external access

   Binding Rails server to 0.0.0.0 allows access from host machine.

3. Step 3: Evaluate options

   Use a volume to mount your app code and run: rails server -b 0.0.0.0 mounts code and binds correctly. Others miss volume or binding, causing no reload or no access.

4. Final Answer:

   Use a volume to mount your app code and run: rails server -b 0.0.0.0 -> Option C

5. Quick Check:

   Volume + bind 0.0.0.0 = live reload + access [OK]

Hint: Mount code volume and bind server to 0.0.0.0 for dev [OK]

Common Mistakes:

• Copying code only once, missing live reload
• Not binding server to 0.0.0.0
• Binding server to localhost disables host access