Always use absolute URLs in Location headers.

This works around docker/docker#15048

test/registry_tests.py

@@ -29,7 +29,7 @@ from jwkest.jwk import RSAKey
 
 import endpoints.decorated # required for side effect
 
-from app import app, storage, instance_keys
+from app import app, storage, instance_keys, get_app_url
 from data.database import close_db_filter, configure, DerivedStorageForImage, QueueItem, Image
 from data import model
 from digest.checksums import compute_simple
@@ -603,7 +603,14 @@ class V2RegistryPushMixin(V2RegistryMixin):
                               expected_code=202, auth='jwt')
 
       upload_uuid = response.headers['Docker-Upload-UUID']
-      location = response.headers['Location'][len(self.get_server_url()):]
+
+      server_hostname = get_app_url()
+      new_upload_location = response.headers['Location']
+      self.assertTrue(new_upload_location.startswith(server_hostname))
+
+      # We need to make this relative just for the tests because the live server test
+      # case modifies the port.
+      location = response.headers['Location'][len(server_hostname):]
 
       # PATCH the image data into the layer.
       if chunks is None: