Automatic merge from submit-queue

make function ReadDockerConfigFile more flexible

In our code, the public function `ReadDockerConfigFile` looks like not enough flexible:
when I want to use this function to get docker config info from a specific path, I have to call  `SetPreferredDockercfgPath`, and then the setting preferredPath will be valid in function  `ReadDockerConfigFile`.  I know in our code, we call  `SetPreferredDockercfgPath` in one place ,then call `ReadDockerConfigFile` in another place, it was not in same context. But it looks like not thread safety. 
 I think if user who use our code want to get docker config from a specific path, it is reasonable to call directly `ReadDockerConfigFile ` with a dockerconfigPath argument, and it can avoid some scenarios that thread is not safety . 
I add a test case for this function.