Transitions the Docker image to use s6 and s6-overlay for process supervision instead of supervisord (#8886)

docker/rootfs/etc/s6-overlay/s6-rc.d/svc-consumer/run

@@ -0,0 +1,10 @@
+#!/command/with-contenv /usr/bin/bash
+# shellcheck shell=bash
+
+cd ${PAPERLESS_SRC_DIR}
+
+if [[ -n "${USER_IS_NON_ROOT}" ]]; then
+	exec python3 manage.py document_consumer
+else
+	exec s6-setuidgid paperless python3 manage.py document_consumer
+fi