Document docker related files in app folder structure

Author: akhilgkrishnan

guides/source/getting_started.md

@@ -177,6 +177,7 @@ of the files and folders that Rails creates by default:
 |config/|Contains configuration for your application's routes, database, and more. This is covered in more detail in [Configuring Rails Applications](configuring.html).|
 |config.ru|Rack configuration for Rack-based servers used to start the application. For more information about Rack, see the [Rack website](https://rack.github.io/).|
 |db/|Contains your current database schema, as well as the database migrations.|
+|Dockerfile|Configuration file for Docker.|
 |Gemfile<br>Gemfile.lock|These files allow you to specify what gem dependencies are needed for your Rails application. These files are used by the Bundler gem. For more information about Bundler, see the [Bundler website](https://bundler.io).|
 |lib/|Extended modules for your application.|
 |log/|Application log files.|
@@ -187,6 +188,7 @@ of the files and folders that Rails creates by default:
 |test/|Unit tests, fixtures, and other test apparatus. These are covered in [Testing Rails Applications](testing.html).|
 |tmp/|Temporary files (like cache and pid files).|
 |vendor/|A place for all third-party code. In a typical Rails application this includes vendored gems.|
+|.dockerignore|This file tells Docker which files it should not copy into the container.|
 |.gitattributes|This file defines metadata for specific paths in a git repository. This metadata can be used by git and other tools to enhance their behavior. See the [gitattributes documentation](https://git-scm.com/docs/gitattributes) for more information.|
 |.gitignore|This file tells git which files (or patterns) it should ignore. See [GitHub - Ignoring files](https://help.github.com/articles/ignoring-files) for more information about ignoring files.|
 |.ruby-version|This file contains the default Ruby version.|