Setting proxy-configuration in config.json was added in commit35f1e301b5(docker 17.07), but never found its way to the documentation. This patch adds some basic information about the feature. Signed-off-by: Sebastiaan van Stijn <github@gone.nl> (cherry picked from commite70d6bcb8a) Signed-off-by: Sebastiaan van Stijn <github@gone.nl>