Hello Guix hackers,

As much as I dislike it, the company I work for has started to use
Docker.  Docker Compose is a special Python program that talks to the
Docker daemon to run many containers at once using configuration from
a "declarative" YAML file (my kingdom for an sexp), and I didn't want
to use pip to install it.  So, here's 8 patches that add it and the
stuff that was missing for it.

I hear someone has a working Go package that hasn't been submitted
yet, so maybe we'll have Docker itself available someday.  That will
be interesting. :)

TIA for review.

- Dave