Add Ruby environment (#223)

Adds a Ruby environment and examples.

environments/ruby/Dockerfile

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+FROM ruby:2.4.1
+
+RUN apt-get update -qq && apt-get install -y build-essential
+
+RUN mkdir /app
+WORKDIR /app
+ADD Gemfile /app/Gemfile
+ADD Gemfile.lock /app/Gemfile.lock
+RUN bundle install
+
+COPY . /app
+
+EXPOSE 8888
+
+ENTRYPOINT ["ruby"]
+CMD ["server.rb"]

environments/ruby/Gemfile

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+# frozen_string_literal: true
+source "https://rubygems.org"
+
+gem "rack"

environments/ruby/Gemfile.lock

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+GEM
+  remote: https://rubygems.org/
+  specs:
+    rack (2.0.3)
+
+PLATFORMS
+  ruby
+
+DEPENDENCIES
+  rack
+
+BUNDLED WITH
+   1.15.1

environments/ruby/README.md

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+# Fission: Ruby Environment
+
+This is the Ruby environment for Fission.
+
+It's a Docker image containing a Ruby 2.4.1 runtime, along with a
+dynamic loader.  A few common dependencies are included in the
+Gemfile.
+
+## Customizing this image
+
+To add package dependencies, edit Gemfile to add what you
+need, and rebuild this image (instructions below).
+
+## Rebuilding and pushing the image
+
+You'll need access to a Docker registry to push the image: you can
+sign up for Docker hub at hub.docker.com, or use registries from
+gcr.io, quay.io, etc.  Let's assume you're using a docker hub account
+called USER.  Build and push the image to the the registry:
+
+```
+   docker build -t USER/ruby-env . && docker push USER/ruby-env
+```
+
+## Using the image in fission
+
+You can add this customized image to fission with "fission env
+create":
+
+```
+   fission env create --name ruby --image USER/ruby-env
+```
+
+Or, if you already have an environment, you can update its image:
+
+```
+   fission env update --name ruby --image USER/ruby-env
+```
+
+After this, fission functions that have the env parameter set to the
+same environment name as this command will use this environment.

environments/ruby/server.rb

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+# frozen_string_literal: true
+require 'rack'
+
+CODEPATH = '/userfunc/user'
+
+module Fission
+  class Request < Rack::Request
+    def headers
+      Hash[
+        *env.select { |k,v| k.start_with?('HTTP_') }
+             .map { |k,v| [k.sub(/\AHTTP_/, '').split('_').map(&:capitalize).join('-'), v] }
+             .sort
+             .flatten
+      ]
+    end
+  end
+
+  class Context
+    attr_reader :env
+
+    def initialize(env)
+      @env = env
+    end
+
+    def request
+      @request ||= Request.new(env)
+    end
+  end
+
+  module Specializer
+    def self.call(env)
+      load CODEPATH
+      Rack::Response.new([], 201).finish
+    rescue
+      Rack::Response.new(['500 Internal Server Error'], 500, {}).finish
+    end
+  end
+
+  module Handler
+    def self.call(env)
+      response = if method(:handler).arity > 0
+        handler(Context.new(env))
+      else
+        handler
+      end
+
+      response.is_a?(Array) ? response : Rack::Response.new([response]).finish
+    rescue
+      Rack::Response.new(['500 Internal Server Error'], 500, {}).finish
+    end
+  end
+end
+
+app = Rack::Builder.new do
+  use Rack::CommonLogger, $stderr
+
+  map "/specialize" do
+    run Fission::Specializer
+  end
+
+  map "/" do
+    run Fission::Handler
+  end
+end
+
+Rack::Handler::WEBrick.run app, Host: '0.0.0.0', Port: 8888