according to https://github.com/pingcap/tidb-docker-compose/issues/106, the image pingcap/tispark:latest can't be downloaded, so fix it to pingcap/tispark:v2.1.1.
In fact, we would like to lead user to use tiup rather than docker-compose which is almost deprecated
according to https://github.com/pingcap/tidb-docker-compose/issues/106, the image
pingcap/tispark:latestcan't be downloaded, so fix it topingcap/tispark:v2.1.1. In fact, we would like to lead user to usetiuprather than docker-compose which is almost deprecated