scheduler: Allow gunicorn logging config to be effective

Refs. swh/infra/sysadm-environment#5327

apps/swh-scheduler/entrypoint.sh

@@ -17,9 +17,14 @@ case "$1" in
         exec swh $@
         ;;
     *)
+        EXTRA_CLI_FLAGS=""
+        if [ ! -z "${SWH_LOG_CONFIG_JSON}" ]; then
+            EXTRA_CLI_FLAGS="--log-config-json ${SWH_LOG_CONFIG_JSON}"
+        fi
         echo Starting the swh-scheduler API server
         exec gunicorn --bind 0.0.0.0:${PORT} \
              --log-level ${SWH_LOG_LEVEL:-INFO} \
+             ${EXTRA_CLI_FLAGS} \
              --threads ${THREADS} \
              --workers ${WORKERS} \
              --reload \

apps/swh-scheduler/requirements.txt

 swh.scheduler
+python-json-logger