Add an index to the docker_image_id for faster lookup

data/database.py

@@ -386,7 +386,7 @@ class Image(BaseModel):
   # to be globally unique we can't treat them as such for permissions and
   # security reasons. So rather than Repository <-> Image being many to many
   # each image now belongs to exactly one repository.
-  docker_image_id = CharField()
+  docker_image_id = CharField(index=True)
   repository = ForeignKeyField(Repository)
 
   # '/' separated list of ancestory ids, e.g. /1/2/6/7/10/