Dockerfile: fix up outdated s6-svscan path (#7880)

## Describe the pull request

Link to the issue: https://github.com/gogs/gogs/issues/7864

docker/start.sh

@@ -79,5 +79,5 @@ fi
 if [ $# -gt 0 ];then
     exec "$@"
 else
-    exec /bin/s6-svscan /app/gogs/docker/s6/
+    exec /usr/bin/s6-svscan /app/gogs/docker/s6/
 fi