feat(buildtrigger): allow use to specify dockerfile

users can only specify the folder and the
  dockerfile must be names "Dockerfile" this allows
  users to specify the file and it can be called
  "Dockerfile" or <some name>.Dockerfile

buildman/component/buildcomponent.py

@@ -1,4 +1,5 @@
 import datetime
+import os
 import time
 import logging
 import json
@@ -122,9 +123,13 @@ class BuildComponent(BaseComponent):
     #  base_image: The image name and credentials to use to conduct the base image pull.
     #   username: The username for pulling the base image (if any).
     #   password: The password for pulling the base image (if any).
+
+    subdir, dockerfile_name = os.path.split(build_config.get('build_subdir', '/Dockerfile'))
+
     build_arguments = {
       'build_package': build_job.get_build_package_url(self.user_files),
-      'sub_directory': build_config.get('build_subdir', ''),
+      'sub_directory': subdir,
+      'dockerfile_name': dockerfile_name,
       'repository': repository_name,
       'registry': self.registry_hostname,
       'pull_token': build_job.repo_build.access_token.code,