swh-scheduler: add SWH_CONFIG_FILENAME and SWH_SCHEDULER_CONFIG_FILE env vars
in all the swh-scheduler related services. Also configure the
CELERY_BROKER_URL env var.
These allow to use the swh-scheduler directly in the container without
any cmdline option, as well as the celery command.
readme: integrate the docker-based development setup from the main's doc
and update several cmdline examples.