added btrfs-prog to the alpine image

Dockerfile

@@ -2,10 +2,8 @@
 FROM postgres:17-alpine
 
 # Install Docker CLI + bash + core utilities
-RUN apk add --no-cache docker-cli bash coreutils
-
-# Create backup directory
-RUN mkdir -p /backups
+RUN apk add --no-cache docker-cli bash coreutils btrfs-prog \
+  && mkdir -p /backups
 
 # Copy backup script
 COPY backup.sh /usr/local/bin/backup.sh