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.