Add a dependency to `docker/swarmkit` in preparation for adding first-class Swarm-wide service management inside the Docker Engine as described in our ROADMAP.md. Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com> Signed-off-by: Jana Radhakrishnan <mrjana@docker.com> Signed-off-by: Victor Vieux <vieux@docker.com> Signed-off-by: Madhu Venugopal <madhu@docker.com> Upstream-commit: 44793049ce1ad364720b4be915a92291d1f808c2 Component: engine