Allow API version specification via env var
Allow API version specification via env var

Hard-coding the API version to '1.18' with the docker-py constructor will
cause the docker-py logic at
https://github.com/docker/docker-py/blob/master/docker/client.py#L143-L146
to always fail, which will cause authentication issues if you're using a
remote daemon using API version 1.19 - regardless of the API version of
the registry.

Allow the user to set the API version via an environment variable. If
the variable is not present, it will still default to '1.18' like it
does today.

Signed-off-by: Reilly Herrewig-Pope <feddfa1b1b567537c5d9b5bb606cac73059ea316@mandiant.com>

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