Docker deployment in Ruby on Rails - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to specify the base image for a Rails app Dockerfile.

Ruby on Rails

FROM [1]

Drag options to blanks, or click blank then click option'

Aubuntu:20.04

Bnode:14

Cpython:3.12

Druby:3.2

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a Node or Python image instead of Ruby.

Using a generic OS image without Ruby installed.

The base image for a Rails app typically uses a Ruby image, such as ruby:3.2.

2fill in blank

medium

Complete the code to copy the Gemfile into the Docker image.

Ruby on Rails

COPY [1] /usr/src/app/Gemfile

Drag options to blanks, or click blank then click option'

AGemfile

Bapp

CDockerfile

Dconfig

Attempts:

3 left

💡 Hint

Common Mistakes

Copying the wrong directory or file.

Copying the Dockerfile instead of Gemfile.

The Gemfile must be copied to the image to install gems.

3fill in blank

hard

Fix the error in the command to install gems in the Dockerfile.

Ruby on Rails

RUN bundle [1]

Drag options to blanks, or click blank then click option'

Ainstall

Bupdate

Cremove

Dinit

Attempts:

3 left

💡 Hint

Common Mistakes

Using bundle update which updates gems but does not install missing ones.

Using bundle remove which removes gems.

The correct command to install gems is bundle install.

4fill in blank

hard

Fill both blanks to expose the correct port and set the working directory.

Ruby on Rails

EXPOSE [1]
WORKDIR [2]

Drag options to blanks, or click blank then click option'

A3000

B/app

C/usr/src/app

D8080

Attempts:

3 left

💡 Hint

Common Mistakes

Exposing port 8080 which is not default for Rails.

Setting working directory to a non-standard path.

Rails apps usually run on port 3000, and the working directory is often set to /usr/src/app inside the container.

5fill in blank

hard

Fill all three blanks to run the Rails server with the correct command and environment.

Ruby on Rails

CMD ["rails", "[1]", "-b", "0.0.0.0", "-e", "[2]", "-p", "[3]"]

Drag options to blanks, or click blank then click option'

Aserver

Bproduction

C3000

Dconsole

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'console' instead of 'server' to start Rails.

Setting environment to 'development' instead of 'production'.

Using wrong port number.

The command runs the Rails server in production mode on port 3000, binding to all interfaces.

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