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/init-system-checks/run

@@ -0,0 +1,15 @@
+#!/command/with-contenv /usr/bin/bash
+# shellcheck shell=bash
+
+declare -r log_prefix="[init-checks]"
+
+# Explicitly run the Django system checks
+echo "${log_prefix} Running Django checks"
+
+cd "${PAPERLESS_SRC_DIR}"
+
+if [[ -n "${USER_IS_NON_ROOT}" ]]; then
+	python3 manage.py check
+else
+	s6-setuidgid paperless python3 manage.py check
+fi