just copy files to mc_data

Dockerfile

@@ -1,11 +1,15 @@
 FROM openjdk:8-slim
 
+RUN apt update && apt install -y rsync
+
 WORKDIR /mc_server
 COPY ./server .
 
 RUN groupadd -r minecraft && useradd --no-log-init -r -g minecraft minecraft
-USER minecraft
 WORKDIR /mc_data
+RUN chown -R minecraft:minecraft .
+
+USER minecraft
 # Copy files only if they don't yet exist (server.jar, server.properties, etc) and start the server
-CMD for file in $(ls /mc_server); do if test ! -s $file; then echo "copying $file" && cp -r /mc_server/$file .; fi; done && \
+CMD cp -r /mc_server/. . && \
 java -server -XX:+UseConcMarkSweepGC -XX:+UseParNewGC -XX:+CMSIncrementalPacing -XX:ParallelGCThreads="$THREADS" -XX:+AggressiveOpts -Xmx"$MAX_RAM"G -Xms"$MIN_RAM"G -jar server.jar nogui