Allow to set log levels via the `SWH_LOG_LEVEL` environment variable
To make it easier to debug code in the Docker stack, allow to set the
general log level through the SWH_LOG_LEVEL
environment variable.
This can then easily be set in docker.override.yml
like so:
swh-lister:
environment:
SWH_LOG_LEVEL: "debug"
Click provides the plumbing to do this easily, see: https://click.palletsprojects.com/en/8.1.x/options/#values-from-environment-variables