Fix Docker::Image.create when `m` is nil

Sometime when pulling for second time the API responds
with nil somewhere, leading to:

       undefined method `[]' for nil:NilClass
     # /var/lib/gems/2.1.0/gems/docker-api-1.13.7/lib/docker/image.rb:102:in `create'

Using reverse_each + find is O(1) compared to O(n) of select + last.
Also splitting it to several lines so there is no call on nil when
there is no image with 'id'.