Closes 6937. Allows setting of docker config dir.

Can now dynamically set the docker config directory through an
environment variable.

export DOCKER_CONFIG=/path/to/docker_config/

Default behavior remains the same, e.g. ~/.docker

Documentation for change added to the https.md docs.

Docker-DCO-1.1-Signed-off-by: James A. Kyle <james@jameskyle.org> (github: jameskyle)