Update Dockerfile

Dockerfile

@@ -20,7 +20,7 @@ RUN apt-get update \
     && chmod +x papermc.sh \
     && chmod +x backup.sh \
     && chmod +x restore.sh
-RUN groupadd -r papermc && adduser papermc
+RUN addgroup -S papermc && adduser papermc
 RUN chown -R ${user} /papermc
 RUN chown -R ${user} /backup