Show the rendered docker file

It can be quite useful (especially while debugging)
to see exactly what the final rendered Dockerfile
actually is; so when it is being rendered, log it
at DEBUG level (so that users running with DEBUG)
can see it.

Change-Id: I28b6f49a7d2f01d155f7885b6ab275c03c4e14af

kolla/image/build.py

@@ -695,8 +695,12 @@ class KollaWorker(object):
                 with open(self.include_footer, 'r') as f:
                     values['include_footer'] = f.read()
             content = template.render(values)
-            with open(os.path.join(path, 'Dockerfile'), 'w') as f:
+            content_path = os.path.join(path, 'Dockerfile')
+            with open(content_path, 'w') as f:
+                LOG.debug("Rendered %s into:", tpl_path)
+                LOG.debug(content)
                 f.write(content)
+                LOG.debug("Wrote it to %s", content_path)
 
     def find_dockerfiles(self):
         """Recursive search for Dockerfiles in the working directory"""