Refactor `Dockerfile`

`Dockerfile` goes beyond the limit of layers supported by docker.
There is an issue [#1171](https://github.com/docker/docker/issues/1171#issuecomment-21353522) in the docker repository about this problem.


Solution for [azk issue #197](https://github.com/azukiapp/azk/issues/197).

MORE INFO:
  https://github.com/azukiapp/azk/issues/197#issuecomment-67875712