Add Ruby/Rack application example with hot reload

integration/examples/ruby/README.md

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+### Example: Ruby/Rack with hot-reload
+
+Simple example based on Ruby/Rack application demonstrating the file synchronization mode.
+
+#### Init
+
+```bash
+skaffold dev
+```
+
+#### Workflow
+
+* Make some changes to `app.rb`:
+    * The file will be synchronized to the cluster
+* Make some changes to `Gemfile`:
+    * The full build/push/deploy process will be triggered, fetching dependencies from `rubygems`
+
+

integration/examples/ruby/backend/Dockerfile

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+FROM ruby:2.7
+
+WORKDIR /app
+ADD Gemfile* /app/
+RUN bundle install
+
+ADD . /app/
+
+CMD ["bundle","exec","puma"]

integration/examples/ruby/backend/Gemfile

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+source 'https://rubygems.org'
+
+gem 'rack'
+gem 'rack-unreloader' # for dynamic reloading
+gem 'puma'

integration/examples/ruby/backend/Gemfile.lock

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+GEM
+  remote: https://rubygems.org/
+  specs:
+    nio4r (2.5.2)
+    puma (4.3.1)
+      nio4r (~> 2.0)
+    rack (2.1.1)
+    rack-unreloader (1.7.0)
+
+PLATFORMS
+  ruby
+
+DEPENDENCIES
+  puma
+  rack
+  rack-unreloader

integration/examples/ruby/backend/app.rb

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+class App
+  def self.call(env)
+    [ 200, {"Content-Type" => "text/html"}, ["Hello Skaffold!"]]
+  end
+end

integration/examples/ruby/backend/config.ru

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+require './app.rb'
+
+require 'rack/unreloader'
+Unreloader = Rack::Unreloader.new{App}
+
+Unreloader.require './app.rb'
+run Unreloader

integration/examples/ruby/k8s/deployment.yaml

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+apiVersion: v1
+kind: Service
+metadata:
+  name: ruby
+spec:
+  ports:
+  - port: 9292
+    targetPort: 9292
+  type: LoadBalancer
+  selector:
+    app: ruby
+---
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: ruby
+spec:
+  selector:
+    matchLabels:
+      app: ruby
+  template:
+    metadata:
+      labels:
+        app: ruby
+    spec:
+      containers:
+      - name: ruby
+        image: ruby-example
+        ports:
+        - containerPort: 9292
+        env:
+          - name: RACK_ENV
+            value: "development"

integration/examples/ruby/skaffold.yaml

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+apiVersion: skaffold/v2alpha1
+kind: Config
+build:
+  artifacts:
+  - image: ruby-example
+    context: backend
+    sync:
+      manual:
+      # Sync only app.rb file, being watch with rack-unreloader
+      - src: 'app.rb'
+        dest: .