Prevent invalid tags on builds

Fixes #1632

endpoints/building.py

@@ -8,6 +8,7 @@ from data import model
 from data.database import db
 from auth.auth_context import get_authenticated_user
 from endpoints.notificationhelper import spawn_notification
+from util.names import escape_tag
 
 
 logger = logging.getLogger(__name__)
@@ -160,7 +161,7 @@ class PreparedBuild(object):
     if self._tags:
       raise Exception('Property tags already set')
 
-    self._tags = list(value)
+    self._tags = [escape_tag(tag, default='latest') for tag in value]
 
   @property
   def build_name(self):