Race condition between task creation and removal

There exists a race condition between task creation and removal
which could causes orphan containers. It happens when a agent task is
handling creation event and is creating a container, a removal event
comes in and cancels the task's context, causing both the creation and
removal fail, but the container is still created by the docker daemon.
The removal fails because when the DELETE request arrives the docker
daemon, the container is still being created.

goroutine (creation)                   task (removal)
---------                              ----

...                                    ...
ctlr.Prepare(ctx)
  r.adapter.createNetworks(ctx)
  ...                                  case <-shutdown
  r.adapter.create(ctx)                  cancel()
  //failed due to context cancelled      tm.ctlr.Remove(ctx)
  //container still createdby daemon     //failed due to "No such
                                         //container" error because
                                         //the container not created
                                         //yet
  ...
  //the container created by docker
  //daemon but becomes orphan

There might be more race conditions between remove and other operations (e.g. Start) for the same task, but no harmful result has been observed so far.

This race issue is discussed in PR #1154, while the orphan container issue was originally reported by https://github.com/docker/docker/issues/24244.