docker_redner: Ease running the command by providing same default as `docker_dev_start --start-redner`.

Factorize postgres connection code.

Previously it would output 21.0.7.dev while after 21.0.7.
It now output 21.0.8.dev as expected.

 docker_dev_start and docker_pylint: do not duplicate mount point for songs when they are correctly defined in python_packages.