go mod: use docker/docker@master instead of docker/engine snapshot

[CpuID]

To avoid conflicts when importing in other projects (dependency hell
between docker/docker vs docker/engine).

On an external project, I'd get errors like this without this PR:

$ go get github.com/docker/libcompose@master
go: finding github.com/docker/libcompose master
go: finding github.com/docker/docker v0.0.0-00010101000000-000000000000
go: github.com/docker/docker@v0.0.0-00010101000000-000000000000: unknown revision 000000000000

cc @vdemeester

---

My 2c: In an ideal world everything would use one repo reference (either docker/engine OR docker/docker), and personally I'd almost suggest docker/engine is cleaner due to no moby/moby rename, but I'm not a decision maker on Docker development and keeping a single canonical import is the safe answer right now across the Go ecosystem.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "fix_docker_docker_mod" git@github.com:CpuID/libcompose.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[CpuID]

Made a mess of this branch, opening a fresh PR.

[CpuID]

New #530