Add a const to allow documenting the environment variable in code. The location
of this const is a bit "unfortunate", due to CLI and Client-config to be spread
over the cli/config, cli/config/configfile, and docker/docker/client packages
(some options are for the client, others for the CLI), and some reorganizing
may be useful for easier consumption.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>