Getting docker cli-plugins in guix

[Jelle Licht]

Hey guix,

Along with many other folks, I find myself in the situation of having to
interact with docker often. The latest docker command line client
supports a way to add new subcommands via a cli-plugins system[1].

This way of working is easy, but relies on building programs and putting
them in kind of random places in either my home directory or the top
level `/usr/...' [2]. I'd much rather have this be self-contained in my
guix-profiles instead.

Patching the docker command line client to look in "the right places"
should IMHO not be too difficult, but finding the right way to go about
this is a different issue.

My current idea is as follows:
- Patch our docker-cli package to respect e.g. `GUIX_DOCKER_PLUGIN_PATH', and 
add it as a search-path
- Attempt to package some docker-cli compatible plugins, which would be
install binaries according to the earlier mentioned search-path.

The advantages of this approach is that it plays nicely with all the
guix commands. The biggest drawback would be that we would have to
maintain a bespoke patch for docker-cli. Another potential problem would
be any new security issues introduced using this simplistic approach.

I'd appreciate some feedback from anyone using docker, because this does
seem like some fairly low-hanging fruit.

- Jelle

[1]: https://github.com/docker/cli/issues/1534
[2]: The suggested installation method implies downloading some random
fat binaries and copying them to your system as-is.