Update gettingstarted.rst
Update gettingstarted.rst

Adding instructions to build docker image
chrislusf/seaweedfs,chrislusf/seaweedfs,k85169336/seaweedfs,chrislusf/seaweedfs,hxiaodon/seaweedfs,hxiaodon/seaweedfs,k85169336/seaweedfs,chrislusf/seaweedfs