Now allows setting of docker config dir.

Can now dynamically set the docker config directory through an
environment variable.

export DOCKER_CONFIG_ROOT=/path/to/docker_config/

Default behavior remains the same, e.g. ~/.docker

Also now uses path/filepath Join method to ensure proper path resolution.
I'm +1 on the feature, but -1 on such a heinous name (but IANTM so take this thought for what it's actually worth, which isn't much).