Consider task ordering in pull command

[jberthold]

The current pull command searches the base image in all target images from the manifest (just like the bash script, the bug originated in the bash version). The correct way is to consider the order, such that the 1st image cannot be based on the 3rd.
Correct the command to

1. only consider earlier targets in the task list to decide whether not to pull
2. report an error if a task uses a base image which is target in a later task