[doc] dashboard docker relative clarity (#4915)
[doc] dashboard docker relative clarity (#4915)

Fixes #4913 

### Motivation

The original document may mislead users into thinking that there is no any ready-made docker image and building one from scratch.
massakam/pulsar,merlimat/pulsar,merlimat/pulsar,merlimat/pulsar,yahoo/pulsar,yahoo/pulsar,massakam/pulsar,yahoo/pulsar,yahoo/pulsar,merlimat/pulsar,massakam/pulsar,yahoo/pulsar,yahoo/pulsar,massakam/pulsar,massakam/pulsar,merlimat/pulsar,massakam/pulsar,merlimat/pulsar