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