Unified Dockerfile for Ruby on Rails projects (http://redgreenrepeat.com/2018/04/20/how-to-conditionally-create-a-file-in-dockerfile-build/)

Dockerfile

FROM ruby:2.5.1-slim

RUN apt-get update && apt-get install -qq -y --no-install-recommends \
    build-essential \
    libsqlite3-dev \
    nodejs \
    sqlite3 \
 && rm -rf /var/lib/apt/lists/*

RUN mkdir /app
WORKDIR /app

COPY Dockerfile Gemfile* .

RUN echo "#!/bin/bash\n" \
         "if [ ! -e \"Gemfile\" ]; then\n" \
         "  echo \"Creating default Gemfile\"\n" \
         "  echo \"ruby '2.5.1'\n" \
         "         source 'https://rubygems.org'\n" \
         "         gem 'rails', '~> 5.1.0'\" > Gemfile\n" \
         "fi\n" > script.sh

RUN chmod +x script.sh
RUN ./script.sh

RUN bundle install

RUN rails new temp_app
RUN rm -rf temp_app