package/docker-engine: bump version to 18.09.6

Additional post-18.09.5 fixes:

Builder:
- Fixed COPY and ADD with multiple <src> to not invalidate cache if
  DOCKER_BUILDKIT=1.moby/moby#38964

Networking:
- Cleaned up the cluster provider when the agent is closed. docker/libnetwork#2354
- Windows: Now selects a random host port if the user does not specify a
  host port.  docker/libnetwork#2369
- --service-cluster-ip-range is now configurable for UCP install.
  docker/orca#10263

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>