run: pull only latest when no tag specified

This makes Docker pull only the image tagged as latest when no tag has
been specified. This makes Docker pull only the image it'll run.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Merge pull request #5841 from unclejack/run_pull_latest

run: pull only latest when no tag specified