docker_build, docker_dev_start: give values to VERSION/VERSION_ARG/SENTRY_RELEASE_TAG/SENTRY_RELEASE closer to what the built image value would be

 Allow setting marabunta_db_user and marabunta_db_password in global configuration file and reuse it if set and they are not provided in the command line

Document tox compatibility with 3.7

This makes the version used clearer.

[skip ci]

Some modules do not compile, so prepend ./ to make it clear the package is a directory

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.