Hi Leo,

as originally released by upstream, Docker looks up auxiliary commands in PATH,
using a Go function called "LookPath".

Our package definition patches a lot of the specific LookPath calls to
refer to inputs by absolute path.

I've booby-trapped the remaining LookPath calls so we won't accidentially
have an internal tool looked up in $PATH.

If we have not forgotten any LookPath calls, there should have been no remaining
LookPath calls and it would not have failed the build.

> .gopath/src/github.com/docker/docker/vendor/github.com/docker/libnetwork/iptables/iptables.go:90:15: undefined: exec.Guix_doesnt_want_LookPath
> .gopath/src/github.com/docker/docker/vendor/github.com/docker/libnetwork/iptables/iptables.go:90:45: invalid character U+005C '\'

Please examine line 90.  It probably has a LookPath line with a new argument we
haven't seen before.

That means we'd have to find out which Guix package has an executable named like
the argument and add a case to the existing LookPath substituter in order to
also substitute it.