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-migrations/run

@@ -0,0 +1,20 @@
+#!/command/with-contenv /usr/bin/bash
+# shellcheck shell=bash
+declare -r log_prefix="[init-migrations]"
+declare -r data_dir="${PAPERLESS_DATA_DIR:-/usr/src/paperless/data}"
+
+(
+	# flock is in place to prevent multiple containers from doing migrations
+	# simultaneously. This also ensures that the db is ready when the command
+	# of the current container starts.
+	flock 200
+	echo "${log_prefix} Apply database migrations..."
+	cd "${PAPERLESS_SRC_DIR}"
+
+	if [[ -n "${USER_IS_NON_ROOT}" ]]; then
+		exec python3 manage.py migrate --skip-checks --no-input
+	else
+		exec s6-setuidgid paperless python3 manage.py migrate --skip-checks --no-input
+	fi
+
+) 200>"${data_dir}/migration_lock"