Avoid unhealthy container when Frigate is stopping (#18021)

docker/main/rootfs/etc/s6-overlay/s6-rc.d/frigate/finish

@@ -25,4 +25,7 @@ elif [[ "${exit_code_service}" -ne 0 ]]; then
   fi
 fi
 
+# used by the docker healthcheck
+touch /dev/shm/.frigate-is-stopping
+
 exec /run/s6/basedir/bin/halt

docker/main/rootfs/etc/s6-overlay/s6-rc.d/prepare/run

@@ -138,5 +138,9 @@ function migrate_db_from_media_to_config() {
     fi
 }
 
+# remove leftover from last run, not normally needed, but just in case
+# used by the docker healthcheck
+rm -f /dev/shm/.frigate-is-stopping
+
 migrate_addon_config_dir
 migrate_db_from_media_to_config