diff --git a/README.md b/README.md
index fae76eb..1d8e0b3 100644
--- a/README.md
+++ b/README.md
@@ -68,10 +68,12 @@ reboot
 
 ## Customize Rootfs (Docker + Kernel Modules)
 Use `customize.sh` to build a writable rootfs with Docker and kernel modules
-preloaded so Docker works out of the box.
+preloaded so Docker works out of the box. Pass the base rootfs as a positional
+argument; the output defaults to `docker-<base filename>` in the same directory
+unless you pass `--out`.
 
 ```
-./customize.sh rootfs-docker.ext4 --size 6G --docker
+./customize.sh ./rootfs.ext4 --size 6G --docker
 ```
 
 Options:
@@ -80,6 +82,7 @@ Options:
 - `--initrd`: initrd path (default: `./wtf/root/boot/initrd.img-6.8.0-94-generic`).
 - `--modules`: kernel modules directory (default: `./wtf/root/lib/modules/6.8.0-94-generic`).
 - `--docker`: install Docker packages into the image.
+- `--out`: output rootfs path (default: `docker-<base filename>`).
 
 After boot, enable NAT and validate Docker:
 ```
diff --git a/customize.sh b/customize.sh
index 1b40abb..91a373e 100755
--- a/customize.sh
+++ b/customize.sh
@@ -7,7 +7,7 @@ log() {
 
 usage() {
   cat <<'EOF'
-Usage: ./customize.sh <output-rootfs> [--size <size>] [--base-rootfs <path>] [--kernel <path>] [--initrd <path>] [--docker] [--modules <dir>]
+Usage: ./customize.sh <base-rootfs> [--out <path>] [--size <size>] [--kernel <path>] [--initrd <path>] [--docker] [--modules <dir>]
 
 Creates a copy of rootfs.ext4, optionally resizes it, boots a VM using the
 copy as a writable rootfs, then applies base configuration and packages.
@@ -46,18 +46,19 @@ BR_IP="172.16.0.1"
 CIDR="24"
 DNS_SERVER="1.1.1.1"
 
+BASE_ROOTFS=""
 OUT_ROOTFS=""
 SIZE_SPEC=""
 INSTALL_DOCKER=0
 MODULES_DIR="$DIR/wtf/root/lib/modules/6.8.0-94-generic"
 while [[ $# -gt 0 ]]; do
   case "$1" in
-    --size)
-      SIZE_SPEC="${2:-}"
+    --out)
+      OUT_ROOTFS="${2:-}"
       shift 2
       ;;
-    --base-rootfs)
-      BASE_ROOTFS="${2:-}"
+    --size)
+      SIZE_SPEC="${2:-}"
       shift 2
       ;;
     --kernel)
@@ -81,8 +82,8 @@ while [[ $# -gt 0 ]]; do
       exit 0
       ;;
     *)
-      if [[ -z "$OUT_ROOTFS" ]]; then
-        OUT_ROOTFS="$1"
+      if [[ -z "$BASE_ROOTFS" ]]; then
+        BASE_ROOTFS="$1"
         shift
       else
         log "unknown option: $1"
@@ -93,7 +94,7 @@ while [[ $# -gt 0 ]]; do
   esac
 done
 
-if [[ -z "$OUT_ROOTFS" ]]; then
+if [[ -z "$BASE_ROOTFS" ]]; then
   usage
   exit 1
 fi
@@ -102,6 +103,12 @@ if [[ ! -f "$BASE_ROOTFS" ]]; then
   log "base rootfs not found: $BASE_ROOTFS"
   exit 1
 fi
+
+if [[ -z "$OUT_ROOTFS" ]]; then
+  base_dir="$(dirname "$BASE_ROOTFS")"
+  base_name="$(basename "$BASE_ROOTFS")"
+  OUT_ROOTFS="${base_dir}/docker-${base_name}"
+fi
 if [[ ! -f "$KERNEL" ]]; then
   log "kernel not found: $KERNEL"
   exit 1
diff --git a/make-rootfs.sh b/make-rootfs.sh
index cf01c38..3d7b3a4 100644
--- a/make-rootfs.sh
+++ b/make-rootfs.sh
@@ -62,8 +62,8 @@ if [[ -z "$BASE_ROOTFS" ]]; then
   fi
 fi
 
-log "building $OUT_ROOTFS from $BASE_ROOTFS"
-exec "$DIR/customize.sh" "$OUT_ROOTFS" \
-  --size "$SIZE_SPEC" \
-  --base-rootfs "$BASE_ROOTFS" \
-  --docker
+  log "building $OUT_ROOTFS from $BASE_ROOTFS"
+  exec "$DIR/customize.sh" "$BASE_ROOTFS" \
+    --out "$OUT_ROOTFS" \
+    --size "$SIZE_SPEC" \
+    --docker