Update entrypoint.sh

about gosu behaviour, it looking for local "existed" UID, i  found it become blank.
That is our issue, so updating our entrypoint to change existed user "youtube" UID and GID to match compose request, will make gosu correctly find use UID:GID as it existed.

backend/entrypoint.sh

@@ -11,6 +11,8 @@ fi
 # chown current working directory to current user
 if [ "$*" = "$CMD" ] && [ "$(id -u)" = "0" ]; then
   find . \! -user "$UID" -exec chown "$UID:$GID" -R '{}' + || echo "WARNING! Could not change directory ownership. If you manage permissions externally this is fine, otherwise you may experience issues when downloading or deleting videos."
+  usermod -u "$UID" "$USER"
+  groupmod -g "$GID" "$USER"
   exec gosu "$UID:$GID" "$0" "$@"
 fi