Stop using directly Docker commands in pythonSetupPyPipeline.groovy file

[ghost]

In the pythonSetupPyPipeline.groovy it is mentioned that we need to use Docker commands directly due to the issue jenkinsci/docker-workflow-plugin#162
There's now a fix available as part of other pull request, which also fixes the 162: jenkinsci/docker-workflow-plugin#180
Hence it should be now fine to stop using the commands directly and use Jenkins features provided by Docker workflow plugin instead.