Fix argument order to pull when tag specified.

[jcrobak]

Fixes an error with pulling a container at a tag (docker requires all flag args before positional args). e.g. with the current impl you'll see:

Expected process to exit with [0], but received '2'       
---- Begin output of docker pull registry  -t 0.6.5 ----       
       STDOUT: 

       STDERR: Usage: docker pull NAME

       Pull an image or a repository from the registry

         -t="": Download tagged image in repository
       ---- End output of docker pull registry  -t 0.6.5 ----
       Ran docker pull registry  -t 0.6.5 returned 2

[bflad]

I should probably go through all of them and verify the correct ordering. Thanks!