Run world archive via docker exec

Stream tar from inside the container to the host, so all server
interaction (rcon + archive) happens through docker exec. Requires
the container to be running; fails fast otherwise.

backup.sh

@@ -1,7 +1,7 @@
 #!/usr/bin/env bash
 # Usage: ./backup.sh [label]
 # Creates backups/world-YYYY-MM-DD-HHMM[-label].tar.gz
-# If the minecraft container is running, flushes world via rcon first.
+# Requires the minecraft container to be running.
 set -euo pipefail
 
 cd "$(dirname "$0")"
@@ -13,17 +13,18 @@ OUT="backups/world-${STAMP}${SUFFIX}.tar.gz"
 
 mkdir -p backups
 
-if docker ps --format '{{.Names}}' | grep -qx minecraft; then
-    echo "Server running — flushing world via rcon..."
+if ! docker ps --format '{{.Names}}' | grep -qx minecraft; then
+    echo "Error: minecraft container is not running." >&2
+    exit 1
+fi
+
+echo "Flushing world via rcon..."
 docker exec minecraft rcon-cli save-off >/dev/null
 docker exec minecraft rcon-cli save-all flush >/dev/null
 trap 'docker exec minecraft rcon-cli save-on >/dev/null || true' EXIT
-else
-    echo "Server not running — archiving world as-is."
-fi
 
-echo "Archiving data/world -> ${OUT}"
-tar -czf "${OUT}" -C data world
+echo "Archiving /data/world -> ${OUT}"
+docker exec minecraft tar -czf - -C /data world > "${OUT}"
 
 echo "Done."
 ls -lh "${OUT}"