feat(build runner): added in context, dockerfile_location

this is a new feature meant to allow people to use any file as
  a dockerfile and any folder as a context directory

endpoints/building.py

@@ -54,6 +54,7 @@ def start_build(repository, prepared_build, pull_robot_name=None):
     'docker_tags': prepared_build.tags,
     'registry': host,
     'build_subdir': prepared_build.subdirectory,
+    'context': prepared_build.context,
     'trigger_metadata': prepared_build.metadata or {},
     'is_manual': prepared_build.is_manual,
     'manual_user': get_authenticated_user().username if get_authenticated_user() else None,
@@ -122,6 +123,7 @@ class PreparedBuild(object):
     self._tags = None
     self._build_name = None
     self._subdirectory = None
+    self._context = None
     self._metadata = None
     self._trigger = trigger
     self._is_manual = None
@@ -224,6 +226,20 @@ class PreparedBuild(object):
 
     self._subdirectory = value
 
+  @property
+  def context(self):
+    if self._context is None:
+      raise Exception('Missing property context')
+
+    return self._context
+
+  @context.setter
+  def context(self, value):
+    if self._context:
+      raise Exception('Property context already set')
+
+    self._context = value
+
   @property
   def metadata(self):
     if self._metadata is None: