Add 3 cases to verify skopeo pulling from docker to ostree works.
The 3 cases include the following:
1: pull dockertar into ostree (custom name)
2: pull dockertar into ostree (default name)
3: pull docker image into ostree(e.g: docker:image:latest)
Also added a small change to ensure checkout are existant if
no installation of containers happens prior.
Closes: #1180
Approved by: giuseppe
