Build image on a separeted workspace, lock building image while another process running

This PR is related to the issue #52

NOTE: This PR has to be merged to master, after merging #82 because this feature depends on the feature implemented in #82 . 
@ainoya I think this will generate directory like `/app/images/app_repo/workspace/<git-commit-id>/<git-commit-id>` as a git repository. Is it right?