Enhancement: Change configuration key for api.version to DOCKER_API_VERSION

[florianproske]

Referring to the issues with Docker Client (like see Issue) I had tried to "inject" the docker client version via environment varaible DOCKER_API_VERSION. As of now, this isn't working, nor it's supposed to work, since the configuration key used currently is "api.version".
For other variables, such as DOCKER_HOST, DOCKER_CONFIG,... the docker variables are already used.
My proposal is, to use DOCKER_API_VERSION instead of api.version.
It would be far more easy to use it, instead of setting the version additionally in a special file, or pass it as java runtime parameter.

And to be sure, DOCKER_API_VERSION is a valid environment variable, as stated in the official documentation: