driver/docker: protect against nil container

Protect against a panic when we attempt to start a container with a name
that conflicts with an existing one.  If the existing one is being
deleted while nomad first attempts to create the container, the
createContainer will fail with `container already exists`, but we get
nil container reference from the `containerByName` lookup, and cause a
crash.

I'm not certain how we get into the state, except for being very
unlucky.  I suspect that this case may be the result of a concurrent
restart or the docker engine API not being fully consistent (e.g. an
earlier call purged the container, but docker didn't free up resources
yet to create a new container with the same name immediately yet).

If that's the case, then re-attempting creation will hopefully succeed,
or we'd at least fail enough times for the alloc to be rescheduled to
another node.

drivers/docker/driver.go

@@ -439,16 +439,21 @@ CREATE:
 			return container, nil
 		}
 
-		// Delete matching containers
-		err = client.RemoveContainer(docker.RemoveContainerOptions{
-			ID:    container.ID,
-			Force: true,
-		})
-		if err != nil {
-			d.logger.Error("failed to purge container", "container_id", container.ID)
-			return nil, recoverableErrTimeouts(fmt.Errorf("Failed to purge container %s: %s", container.ID, err))
-		} else {
-			d.logger.Info("purged container", "container_id", container.ID)
+		// Purge conflicting container if found.
+		// If container is nil here, the conflicting container was
+		// deleted in our check here, so retry again.
+		if container != nil {
+			// Delete matching containers
+			err = client.RemoveContainer(docker.RemoveContainerOptions{
+				ID:    container.ID,
+				Force: true,
+			})
+			if err != nil {
+				d.logger.Error("failed to purge container", "container_id", container.ID)
+				return nil, recoverableErrTimeouts(fmt.Errorf("Failed to purge container %s: %s", container.ID, err))
+			} else {
+				d.logger.Info("purged container", "container_id", container.ID)
+			}
 		}
 
 		if attempted < 5 {