First Pass

Dockerfile

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+FROM ruby:latest
+
+# throw errors if Gemfile has been modified
+RUN bundle config --global frozen 1
+
+WORKDIR /usr/src/app/
+
+COPY Gemfile Gemfile.lock ./
+RUN bundle install
+
+ADD . /usr/src/app/
+
+EXPOSE 4000
+
+CMD ["ruby", "/usr/src/app/helloworld.rb"]

Gemfile

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+# frozen_string_literal: true
+
+source "https://rubygems.org"
+
+git_source(:github) {|repo_name| "https://github.com/#{repo_name}" }
+
+# gem "rails"
+gem "sinatra"

Gemfile.lock

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+GEM
+  remote: https://rubygems.org/
+  specs:
+    mustermann (3.0.0)
+      ruby2_keywords (~> 0.0.1)
+    rack (2.2.7)
+    rack-protection (3.0.6)
+      rack
+    ruby2_keywords (0.0.5)
+    sinatra (3.0.6)
+      mustermann (~> 3.0)
+      rack (~> 2.2, >= 2.2.4)
+      rack-protection (= 3.0.6)
+      tilt (~> 2.0)
+    tilt (2.2.0)
+
+PLATFORMS
+  x86_64-linux
+
+DEPENDENCIES
+  sinatra
+
+BUNDLED WITH
+   2.2.26

helloworld.rb

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+require 'sinatra'
+
+set :port, 4000
+set :bind, '0.0.0.0'
+
+get '/' do
+    'Hello World!'
+end