Dockerfile

FROM ruby:2.3-alpine

# Install dependencies required to run Rails as well as RubyGems.
RUN apk --update add --virtual build-dependencies curl-dev ruby-dev build-base \
    openssl-dev libc-dev linux-headers zlib-dev libxml2-dev libxslt-dev tzdata \
    yaml-dev postgresql-dev nodejs imagemagick git file

# Configure the main working directory. This is the base 
# directory used in any further RUN, COPY, and ENTRYPOINT 
# commands.
RUN mkdir -p /app 
WORKDIR /app

# Copy the Gemfile as well as the Gemfile.lock and install 
# the RubyGems. This is a separate step so the dependencies 
# will be cached unless changes to one of those two files 
# are made.
COPY Gemfile Gemfile.lock ./ 
RUN gem install bundler && bundle install --jobs 20 --retry 5

# Copy the main application.
COPY . ./

# Expose port 3000 to the Docker host, so we can access it 
# from the outside.
EXPOSE 3000

# Configure an entry point, so we don't need to specify 
# "bundle exec" for each of our commands.
ENTRYPOINT ["bundle", "exec"]

# The main command to run when the container starts. Also 
# tell the Rails dev server to bind to all interfaces by 
# default.
CMD ["rails", "server", "-b", "0.0.0.0"]