go mod: use docker/docker@master instead of docker/engine snapshot (Take 2)

[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/[email protected]: unknown revision 000000000000

cc @vdemeester

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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.