Update Dockerfile

fixed stupid mistake (forgotten to add package to docker container ^^)

Dockerfile

@@ -11,7 +11,7 @@ ADD papermc.sh .
 ADD backup.sh .
 ADD restore.sh .
 RUN apt-get update \
-    && apt-get install wget curl jq webp screen -y \
+    && apt-get install wget curl jq webp screen grep -y \
     && rm -rf /var/lib/apt/lists/* \
     && mkdir /papermc \
     && mkdir /backup