Pull latest tag by default

This was changed in Docker recently:

https://github.com/docker/docker/pull/7759

This means we aren't pulling loads of tags when we only use the
latest.

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
This `_get_container_create_options()` is already doing a lot of work unrelated to creating the image name with a tag (none of which is used by `pull()`). Maybe this could be a new function `get_image_name()`, called from both `pull` and `_get_container_create_options` ?