added restore script for backup volumes

extra/docker_volume_backupper/docker_volume_restore.sh

@@ -0,0 +1,72 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+if [ $# -lt 1 ]; then
+  echo "Usage: $0 <backup-file1.tar.gz|.tgz|.tar> [backup-file2 ...]"
+  exit 1
+fi
+
+extract_volume_name() {
+  local f="$1"
+  local b
+  b="$(basename "$f")"
+  # strip known extensions
+  b="${b%.tar.gz}"
+  b="${b%.tgz}"
+  b="${b%.tar}"
+  # strip trailing _YYYY-MM-DD or -YYYY-MM-DD (optionally followed by anything)
+  b="$(echo "$b" | sed -E 's/([_-][0-9]{4}-[0-9]{2}-[0-9]{2})([_-].*)?$//')"
+  printf '%s' "$b"
+}
+
+volume_exists() {
+  docker volume inspect "$1" >/dev/null 2>&1
+}
+
+tar_flag_for() {
+  case "$1" in
+    *.tar.gz|*.tgz) echo "xzf" ;;
+    *.tar)          echo "xf"  ;;
+    *)              echo "xzf" ;; # default assume gzip
+  esac
+}
+
+for backup in "$@"; do
+  if [ ! -f "$backup" ]; then
+    echo "SKIP: file not found: $backup"
+    continue
+  fi
+
+  vol="$(extract_volume_name "$backup")"
+  if [ -z "$vol" ]; then
+    echo "SKIP: could not infer volume name from $backup"
+    continue
+  fi
+
+  if ! volume_exists "$vol"; then
+    echo "SKIP: docker volume does not exist: $vol  (from $backup)"
+    continue
+  fi
+
+  absdir="$(realpath "$(dirname "$backup")")"
+  base="$(basename "$backup")"
+  tarflags="$(tar_flag_for "$backup")"
+  cname="restore_${vol}_$(date +%s)_$$"
+
+  echo "RESTORE: $base -> volume $vol"
+
+  # restore into volume: wipe existing contents then extract
+  docker run --rm \
+    --name "$cname" \
+    -v "$vol":/data \
+    -v "$absdir":/backup \
+    alpine sh -c "
+      set -e
+      cd /data
+      rm -rf ./* .??* 2>/dev/null || true
+      tar ${tarflags} \"/backup/${base}\" -C /data
+    "
+
+  echo "OK: restored $vol"
+done
+