Make the error more specific by stating that it's caused by a specific
environment variable and not an environment as a whole.
Also don't escape the variable to make it more readable.
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>
(cherry picked from commit 012b77952e)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
b61b5a9878