Version pin github.com/docker/docker

Change the source for `github.com/docker/docker` to
`github.com/docker/engine` and version pin it to the latest tag `18.09`
this would match the version of `github.com/docker/cli`. There is a good
explantion as to why use `github.com/docker/engine` as a source instead
of `github.com/moby/moby` in
https://github.com/moby/moby/issues/38063#issuecomment-431324613.

Some other related issues as to why using `github.com/docker/engine` is
ideal:
- https://github.com/moby/moby/issues/38507
- https://github.com/moby/moby/issues/38134
- https://github.com/moby/moby/issues/37683
- https://github.com/moby/moby/issues/37529
- https://github.com/moby/moby/issues/38063

https://gitlab.com/gitlab-org/gitlab-runner/issues/3488