Fix argument order to pull when tag specified.

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
```