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)

api/client/commands.go

@@ -1840,6 +1840,10 @@ func (cli *DockerCli) CmdRun(args ...string) error {
 
 		v := url.Values{}
 		repos, tag := utils.ParseRepositoryTag(config.Image)
+		// pull only the image tagged 'latest' if no tag was specified
+		if tag == "" {
+			tag = "latest"
+		}
 		v.Set("fromImage", repos)
 		v.Set("tag", tag)