Could not find gem rails - ruby-on-rails

Question

I am trying to build a docker container but getting the following error message:
Step 8/12 : RUN bundle binstubs bundler --force
---> Running in ed94b127974b
Could not find gem 'rails (>= 5.1.5, ~> 5.1)' in any of the gem sources listed
in your Gemfile.
ERROR: Service 'dockerzon' failed to build: The command '/bin/sh -c bundle binstubs bundler --force' returned a non-zero code: 7
I have tried with different versions of rails in my Gemfile with no success! This is what I have in my Gemfile:
source 'https://rubygems.org'
# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
# gem 'rails', '4.2.6'
# gem 'rails', '>= 5.1.5'
gem 'rails', '~> 5.1', '>= 5.1.5'
# Use sqlite3 as the database for Active Record
# gem 'sqlite3'
# Use SCSS for stylesheets
gem 'sass-rails', '~> 5.0.7'
# Use Uglifier as compressor for JavaScript assets
gem 'uglifier', '>= 4.1.6'
# Use CoffeeScript for .coffee assets and views
gem 'coffee-rails', '~> 4.2.2'
# See https://github.com/rails/execjs#readme for more supported runtimes
# gem 'therubyracer', platforms: :ruby
# Use jquery as the JavaScript library
gem 'jquery-rails'
# Turbolinks makes following links in your web application faster. Read more: https://github.com/rails/turbolinks
gem 'turbolinks'
# Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder
gem 'jbuilder', '~> 2.7'
# bundle exec rake doc:rails generates the API under doc/api.
gem 'sdoc', '~> 1.0.0', group: :doc
# Use ActiveModel has_secure_password
# gem 'bcrypt', '~> 3.1.7'
# Use Unicorn as the app server
# gem 'unicorn'
# Use Capistrano for deployment
# gem 'capistrano-rails', group: :development
group :development, :test do
# Call 'byebug' anywhere in the code to stop execution and get a debugger console
gem 'byebug'
end
group :development do
# Access an IRB console on exception pages or by using <%= console %> in views
gem 'web-console', '~> 3.5.1'
# Spring speeds up development by keeping your application running in the background. Read more: https://github.com/rails/spring
gem 'spring'
end
gem 'pg', '~> 1.0.0'
gem 'redis-rails', '~> 5.0.2'
gem 'sidekiq', '~> 5.1.1'
gem 'puma', '~> 3.11.2'
gem 'rack-timeout', '~> 0.4.2'
and this is my Dockerfile:
# Use the barebones version of Ruby 2.5
FROM ruby:2.5-slim
# Optionally set a maintainer name to let people know who made this image.
# Install dependencies:
# - build-essential: To ensure certain gems can be compiled
# - nodejs: Compile assets
# - libpq-dev: Communicate with postgres through the postgres gem
RUN apt-get update && apt-get install -qq -y --no-install-recommends \
build-essential nodejs libpq-dev
# Set an environment variable to store where the app is installed to inside
# of the Docker image. The name matches the project name out of convention only.
ENV INSTALL_PATH /dockerzon
RUN mkdir -p $INSTALL_PATH
# This sets the context of where commands will be ran in and is documented
# on Docker's website extensively.
WORKDIR $INSTALL_PATH
# Ensure gems are cached and only get updated when they change. This will
# drastically increase build times when your gems do not change.
COPY Gemfile Gemfile
# We want binstubs to be available so we can directly call sidekiq and
# potentially other binaries as command overrides without depending on
# bundle exec.
# RUN bundle install --binstubs
RUN bundle binstubs bundler --force
# Copy in the application code from your work station at the current directory
# over to the working directory.
COPY . .
# Provide a dummy DATABASE_URL to Rails so it can pre-compile assets.
RUN bundle exec rake RAILS_ENV=production DATABASE_URL=postgresql://user:pass#127.0.0.1/dbname SECRET_TOKEN=dummytoken assets:precompile
# Ensure the static assets are exposed through a volume so that nginx can read
# in these values later.
VOLUME ["$INSTALL_PATH/public"]
# The default command that gets ran will be to start the Puma server.
CMD bundle exec puma -C config/puma.rb

Answer 1

Try to update the version of your RubyGems:
gem update --system
Then change the line in Gemfile to:
gem 'rails', '~> 5.1.5'
Now run bundler to install the gems:
bundle install

Answer 2

Add below lines in your Dockerfile
RUN gem install rails
ruby:2.5-slim this docker image doesn't contain rails. It only contains ruby. You can't install rails using Gemfile. You have to install rails before bundling your gemfile.