Switch to send full AuthConfig object for build action

In order to support the docker API for version 1.7+, this command
changes the way the `X-Registry-Config` header is sent when attempting
to build an image.

docker/auth/auth.py

@@ -102,12 +102,6 @@ def encode_header(auth):
     return base64.b64encode(auth_json)
 
 
-def encode_full_header(auth):
-    """ Returns the given auth block encoded for the X-Registry-Config header.
-    """
-    return encode_header({'configs': auth})
-
-
 def parse_auth(entries):
     """
     Parses authentication entries

docker/client.py

@@ -139,7 +139,7 @@ class Client(clientbase.ClientBase):
             if self._auth_configs:
                 if headers is None:
                     headers = {}
-                headers['X-Registry-Config'] = auth.encode_full_header(
+                headers['X-Registry-Config'] = auth.encode_header(
                     self._auth_configs
                 )