An annotated Dockerfile for Rails, running Ruby 2.2.5

Dockerfile

# Our base image is Ruby 2.2; it will run on Amazon Linux.
FROM ruby:2.2

# This isn't required, but give yourself some credit
MAINTAINER Your Name Here <Your Email Here>

# Install packages
RUN apt-get update && apt-get install -y \
    git \
    nodejs \
    tzdata

# Copy your application into the container.
COPY . .

# Declare arguments that will vary by environment so we can pass them in
ARG ASSET_HOST
ARG SECRET_KEY

# Build your application.
RUN \
    # Install application gems.
    bundle install --jobs 4 --without development test --with production && \
    # Precompile Rails assets, using the secret keys
    bundle exec rake ASSET_HOST=${ASSET_HOST} SECRET_KEY=${SECRET_KEY} RAILS_ENV=production assets:precompile && \
    # Clean up build package
    rm -rf /usr/local/lib/ruby/gems/*/cache/* && \
    rm -rf ~/.gem

# Run the application.
CMD bin/rails server --port 3000