Send push information to trust code out-of-band

The trust code used to parse the console output of `docker push` to
extract the digest, tag, and size information and determine what to
sign. This is fragile and might give an attacker control over what gets
signed if the attacker can find a way to influence what gets printed as
part of the push output.

This commit sends the push metadata out-of-band. It introduces an `Aux`
field in JSONMessage that can carry application-specific data alongside
progress updates. Instead of parsing formatted output, the client looks
in this field to get the digest, size, and tag from the push.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>
Upstream-commit: 65370be888d940899593a001024f53d6b83b4bb0
Component: engine

components/engine/api/client/pull.go

@@ -83,5 +83,5 @@ func (cli *DockerCli) imagePullPrivileged(authConfig types.AuthConfig, imageID,
 	}
 	defer responseBody.Close()
 
-	return jsonmessage.DisplayJSONMessagesStream(responseBody, cli.out, cli.outFd, cli.isTerminalOut)
+	return jsonmessage.DisplayJSONMessagesStream(responseBody, cli.out, cli.outFd, cli.isTerminalOut, nil)
 }