Remove --rm from docker run commands

Trying to avoid "Failed to destroy btrfs snapshot" errors on CircleCI

test/lib/docker_helpers.bash

@@ -56,5 +56,5 @@ function docker_tcp {
 		--expose 2375 \
 		-v /var/run/docker.sock:/var/run/docker.sock \
 		rancher/socat-docker
-	docker run --rm --link "$container_name:docker" docker version
+	docker run --link "$container_name:docker" docker version
 }