Setting proxy-configuration in config.json was added in
commit 35f1e301b573f057937c0d3f3232e4f03cd21a5d (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>
e70d6bcb8a