Update to Rails 5, Ruby 2.3.3, remove therubyracer (#718)

* Update to Rails 5, Ruby 2.3.3, remove therubyracer

Update packages to latest versions. The actual nodejs is installed on line 18. Changed to a general comment regarding the need to rebuild images when adding gems.

* update rails welcome screenshot to 5.0.0.1

compose/rails.md

@@ -14,7 +14,7 @@ dependencies, you'll need to define exactly what needs to be included in the
 container. This is done using a file called `Dockerfile`. To begin with, the
 Dockerfile consists of:
 
-    FROM ruby:2.2.0
+    FROM ruby:2.3.3
     RUN apt-get update -qq && apt-get install -y build-essential libpq-dev nodejs
     RUN mkdir /myapp
     WORKDIR /myapp
@@ -30,7 +30,7 @@ how to write Dockerfiles, see the [Docker user guide](/engine/tutorials/dockerim
 Next, create a bootstrap `Gemfile` which just loads Rails. It'll be overwritten in a moment by `rails new`.
 
     source 'https://rubygems.org'
-    gem 'rails', '4.2.0'
+    gem 'rails', '5.0.0.1'
 
 You'll need an empty `Gemfile.lock` in order to build our `Dockerfile`.
 
@@ -99,12 +99,7 @@ If you are running Docker on Mac or Windows, you should already have ownership
 of all files, including those generated by `rails new`. List the files just to
 verify this.
 
-Uncomment the line in your new `Gemfile` which loads `therubyracer`, so you've
-got a Javascript runtime:
-
-    gem 'therubyracer', platforms: :ruby
-
-Now that you've got a new `Gemfile`, you need to build the image again. (This,
+If you edit `Gemfile` at this point or later, you will need to build the image again. (This,
 and changes to the Dockerfile itself, should be the only times you'll need to
 rebuild.)