Hello Guix!

Before this change, attempting to start a docker registry such as with
the following command[0]:

--8<---------------cut here---------------start------------->8---
docker run -d -p 5000:5000 --restart=always --name registry registry:2
--8<---------------cut here---------------end--------------->8---

Would give the following error:

--8<---------------cut here---------------start------------->8---
/gnu/store/dzaijl53fcd3jhkpd70vsf4cnvv10ywj-docker-cli-18.09.3/bin/docker:
Error response from daemon: driver failed programming external
connectivity on endpoint registry
(709754084a9e208c32075e47ea9584296a6f274deeef08283d0de9c9a5161112):
exec: "docker-proxy": executable file not found in $PATH.
--8<---------------cut here---------------end--------------->8---

This series of patches adds docker-libnetwork-cmd-proxy and a few new go
packages it required.

The docker service is modified to enable (or disable) using a 'docker-proxy'.

Thanks!

Maxim