fix: parse_image_id: Process "docker build" output in reverse line or…

…der (#400)

When using a custom dockerfile which installs Python modules using pip,
the output will sometimes contain the line:

Successfully built module_name

Since this matches the regex, it causes the module name to be
incorrectly identified as the image ID, which will cause kitchen-docker
to fail the "docker run" as it will not have a valid Docker image ID.

This commit processes the "docker build" output in reverse line order,
which will ensure that the correct line is matched.

Co-authored-by: Erik Johnson <erik.johnson@centurylink.com>

lib/kitchen/docker/helpers/image_helper.rb

@@ -25,7 +25,7 @@ module ImageHelper
         include Kitchen::Docker::Helpers::ContainerHelper
 
         def parse_image_id(output)
-          output.each_line do |line|
+          output.split("\n").reverse_each do |line|
             if line =~ /writing image (sha256:[[:xdigit:]]{64})(?: \d*\.\ds)? done/i
               img_id = line[/writing image (sha256:[[:xdigit:]]{64})(?: \d*\.\ds)? done/i,1]
               return img_id