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.
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