Docker deployment in Ruby on Rails - Step-by-Step Execution

Concept Flow - Docker deployment

Write Dockerfile

↓

Build Docker Image

↓

Run Container from Image

↓

App Starts Inside Container

↓

Access App via Browser or API

↓

Stop or Restart Container

This flow shows how a Rails app is packaged into a Docker image, run as a container, and accessed.

Execution Sample

Ruby on Rails

FROM ruby:3.2
WORKDIR /app
COPY Gemfile* ./
RUN bundle install
COPY . .
CMD ["rails", "server", "-b", "0.0.0.0"]

This Dockerfile builds a Docker image for a Rails app and starts the Rails server.

Execution Table

Step	Action	Details	Result
1	Read Dockerfile	FROM ruby:3.2	Base image set to Ruby 3.2
2	Set working directory	WORKDIR /app	Working directory inside container is /app
3	Copy Gemfile	COPY Gemfile* ./	Gemfile and Gemfile.lock copied to /app
4	Install gems	RUN bundle install	Gems installed inside image
5	Copy app files	COPY . .	All app files copied to /app
6	Set start command	CMD ["rails", "server", "-b", "0.0.0.0"]	Rails server will start on container launch
7	Build image	docker build -t myrailsapp .	Docker image 'myrailsapp' created
8	Run container	docker run -p 3000:3000 myrailsapp	Container runs, Rails server listens on port 3000
9	Access app	Open http://localhost:3000	Rails app homepage loads in browser
10	Stop container	Ctrl+C or docker stop	Container stops, app no longer accessible

💡 Container stops when user stops it or system shuts down

Variable Tracker

Variable	Start	After Step 3	After Step 5	After Step 8	Final
Docker Image	None	Base Ruby image	Image with app files and gems	Image running as container	Stopped container
Container	None	None	None	Running Rails server	Stopped

Key Moments - 3 Insights

Why do we copy Gemfile separately before copying all files?

Copying Gemfile first allows Docker to cache the bundle install step if Gemfile hasn't changed, speeding up rebuilds (see execution_table steps 3 and 4).

Why do we bind Rails server to 0.0.0.0 instead of localhost?

Binding to 0.0.0.0 makes the server listen on all network interfaces inside the container, allowing access from outside (step 6).

What happens if we don't expose port 3000 when running the container?

Without port mapping (step 8), the app inside the container won't be reachable from the host machine's browser.

Visual Quiz - 3 Questions

Look at the execution table, what is the result after step 4?

A Gems installed inside image

B App files copied to /app

C Rails server started

D Docker image created

💡 Hint

Check the 'Result' column for step 4 in the execution_table

At which step does the Rails server start running inside the container?

A Step 6

B Step 8

C Step 7

D Step 9

💡 Hint

Look for when the container runs and the server listens on port 3000

If we change the CMD to bind Rails server to localhost (127.0.0.1), what happens?

A Container fails to start

B App is accessible from host machine

C App is not accessible from host machine

D Docker image build fails

💡 Hint

Refer to key_moments about binding address in step 6

Concept Snapshot

Docker deployment for Rails:
- Write Dockerfile with base Ruby image
- Copy Gemfile, run bundle install for caching
- Copy app files
- Set CMD to start Rails server binding 0.0.0.0
- Build image with docker build
- Run container with port mapping
- Access app via localhost:3000
- Stop container to end app

Full Transcript

Docker deployment for a Rails app involves writing a Dockerfile that starts from a Ruby base image. We set the working directory and copy the Gemfile first to install gems efficiently. Then we copy the rest of the app files. The CMD instruction starts the Rails server binding to 0.0.0.0 so it listens on all interfaces inside the container. We build the Docker image using 'docker build' and run it with 'docker run' mapping port 3000 to access the app from the host browser. Stopping the container stops the app. This process packages the Rails app in a container for easy deployment and consistent environment.

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