Add output variable to ECRPullImage task

**Description**
It would be nice for the ECRPullImage task to output the image that it pulls, so that it can be used in downstream tasks. This could be done in a similar manner to the ECRPushImage task output variable.

Start by locating the ECRPullImage task and compare its output handling with the ECRPushImage task output variable mentioned in the issue. The work is done when ECRPullImage exposes the pulled image so downstream tasks can consume it, with any relevant task tests updated or added.

Written by the indexing model from the issue text.