4196: arv-keepdocker uses Docker 1.0 pull syntax.

`docker pull -t` was the old way of pulling an image with a specific
tag.  Docker 1.0 introduced `docker pull repo:tag`, and Docker 1.2
removes support for `-t` completely.  We're well-standardized on 1.1+
at this point, and considering an upgrade to 1.2+, so now's the time
to switch.

Closes #4196.