Initial commit for docker cookbook.

This commit refs #194.
Install dependenpt packages for docker.

Check if the docker support the target platform,
and if it does docker will be installed.

This commit refs #194.
Install docker.

This commit refs #194.
Pull ubuntu docker image.

This commit refs #194.
Merge branch 'id/194' into dev
Correct gem package name.

  - Before: bundle
  - After : bundler

This commit refs #197.
Merge branch 'id/197' into dev
Enables the user to specify the group of /home/user/.rbenv.

jenkins package will add jenkins user
and the owner and group of jenkins home directory is: jenkins and nogroup.

Before this commit,
this cookbook assumes that the owner and group
of /path/to/.rbenv is the specified user.
For example, if you install Ruby under kazu634's home directory,
its .rbenv directory owner and group owner is: kazu634:kazu634.

This will not work for jenkins case.

After this commit,
you can specify the owner and group owner of /home/user/.rbenv separately.

This commit refs #198.
Modify README.md to illustrate how to use this cookbook.

This commit fixes #198.
Merge branch 'id/198' into dev