Correct document for building the python SDK harness container

In the runner document, it is described that we can use the command: `./gradlew :sdks:python:container:docker`
to Build the SDK harness container, see (https://beam.apache.org/documentation/runners/flink/).

However, the docker config has been removed with the latest python3 docker related commit [1] the command would failed with the following error message.

 > Task :sdks:python:container:docker FAILED

 FAILURE: Build failed with an exception.

 * What went wrong:
 Execution failed for task ':sdks:python:container:docker'.
 > name is a required docker configuration item.

I think we should also adapt the document with command: `./gradlew :sdks:python:container:py2:docker`? Or add the config when run `:sdks:python:container:docker` auto run all the python version docker?

What do you think?

[1] https://github.com/apache/beam/commit/47feeafb21023e2a60ae51737cc4000a2033719c#diff-1bc5883bcfcc9e883ab7df09e4dcddb0L63