Add front page for waiting while building Docker image and running it #6

I changed code to add front page while building Docker image. #6 

1. Divide build module to `builder/`
2. Showing font page while building Docker image and run it
  * Return a html to show build log. It communicates with `build_server.rb` in proxy server by Websocket
  * Add `build_server.rb` to handle this. It gets request from front page then launches some build tasks. Logs are sent from `build_server.rb` through Websocket. 

@ainoya Could you check it out? Thanks :beer:
For downloading gems faster, add `gem` command to `--no-doc --no-ri` option.