RepoRootPath must be absolute path

Fix #252

Using strings.HasPrefix(...) will cause a misjudgement

`strings.HasPrefix(access.RepoName, uname)` can't handle the situation which like following in access table.

	user_name | repo_name
	----------+-------------
	toby      | toby/blog
	toby      | toby/test
	toby      | tobyzxj/blog
	toby      | tobyzxj/test

use 'docker.io' command instead of 'docker'