Docker deployment in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when you run this Dockerfile for a Rails app?

Consider this Dockerfile snippet for a Rails app:

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

What will be the result when you build and run this container?

Ruby on Rails

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

A The container builds but never runs because CMD is missing an entrypoint.

B The container runs but the Rails server binds only to localhost, so it's inaccessible outside.

C The container starts a Rails server accessible on all network interfaces inside the container.

D The container fails because Gemfile is not copied before bundle install.

💡 Hint

Think about what the CMD command does and the effect of the -b 0.0.0.0 option.

The CMD runs the Rails server binding to 0.0.0.0, which means it listens on all interfaces inside the container, making it accessible from outside if ports are mapped.

📝 Syntax

intermediate

Identify the syntax error in this Docker Compose service for Rails

Look at this Docker Compose service snippet:

version: '3.8'
services:
  web:
    build: .
    ports:
      - "3000:3000"
    volumes:
      - .:/app
    command: rails server -b 0.0.0.0
    environment:
      - DATABASE_URL=postgres://user:pass@db:5432/mydb

What is the syntax error here?

Ruby on Rails

version: '3.8'
services:
  web:
    build: .
    ports:
      - "3000:3000"
    volumes:
      - .:/app
    command: rails server -b 0.0.0.0
    environment:
      - DATABASE_URL=postgres://user:pass@db:5432/mydb

A The environment variable DATABASE_URL must be under quotes.

B The volumes syntax is invalid; it should use double slashes.

C The ports mapping should be integers, not strings.

D The command should be an array: command: ["rails", "server", "-b", "0.0.0.0"]

💡 Hint

YAML allows strings but Docker Compose recommends arrays for commands with arguments.

Using an array for the command ensures arguments are passed correctly without shell interpretation issues.

🔧 Debug

advanced

Why does the Rails app fail to connect to the database in this Docker setup?

You have a Docker Compose file with services 'web' and 'db'. The Rails app fails to connect to the database with a connection refused error.

services:
  web:
    build: .
    depends_on:
      - db
    environment:
      DATABASE_URL: postgres://user:pass@db:5432/mydb
  db:
    image: postgres:14
    environment:
      POSTGRES_USER: user
      POSTGRES_PASSWORD: pass
      POSTGRES_DB: mydb

What is the most likely cause?

Ruby on Rails

services:
  web:
    build: .
    depends_on:
      - db
    environment:
      DATABASE_URL: postgres://user:pass@db:5432/mydb
  db:
    image: postgres:14
    environment:
      POSTGRES_USER: user
      POSTGRES_PASSWORD: pass
      POSTGRES_DB: mydb

A The web service tries to connect before the db service is ready to accept connections.

B The DATABASE_URL is missing the protocol prefix.

C The db service lacks a volume for data persistence causing immediate failure.

D The depends_on key is invalid and ignored by Docker Compose.

💡 Hint

depends_on only waits for container start, not readiness.

depends_on ensures the db container starts but does not wait for the database to be ready. Rails tries to connect too early, causing connection refused.

❓ state_output

advanced

What is the effect of this Dockerfile layer caching strategy on Rails app build time?

Given this Dockerfile snippet:

FROM ruby:3.1
WORKDIR /app
COPY Gemfile Gemfile.lock ./
RUN bundle install
COPY . .
RUN rails assets:precompile
CMD ["rails", "server", "-b", "0.0.0.0"]

How does this order affect build time when only app code changes?

Ruby on Rails

FROM ruby:3.1
WORKDIR /app
COPY Gemfile Gemfile.lock ./
RUN bundle install
COPY . .
RUN rails assets:precompile
CMD ["rails", "server", "-b", "0.0.0.0"]

A Every build reruns bundle install because Gemfile is copied after app code.

B Only the last two layers rebuild, so bundle install is cached, speeding up builds.

C Assets precompile runs before bundle install causing errors.

D The CMD layer caches the entire build, so no rebuilds happen.

💡 Hint

Docker caches layers and reuses them if previous steps don't change.

Since Gemfile and Gemfile.lock are copied before bundle install, if they don't change, that layer is cached. Copying app code later means only app code changes trigger rebuilds after bundle install.

🧠 Conceptual

expert

Which Docker networking mode allows a Rails container to share the host's network stack?

You want your Rails Docker container to use the host machine's network directly, without port mapping. Which Docker network mode achieves this?

A host

B none

C bridge

D overlay

💡 Hint

Think about which mode removes network isolation between container and host.

The 'host' network mode makes the container share the host's network stack, so no port mapping is needed and the container uses host IP addresses directly.

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