chore: split compose into separate files

- Split compose.yml into two files: compose.yml for running and compose.prepare.yml for golden image preparation
- Rename env vars STORAGE->STORAGE_DIR and ISO->ISO_FILE to avoid conflicts with internal script variables
- Remove unused VERSION env var (hardcoded in scripts)
- Update README with new command syntax

compose.prepare.yml

@@ -0,0 +1,19 @@
+services:
+  prepare-windows:
+    image: windows-local:latest
+    container_name: prepare-windows
+    environment:
+      ISO_FILE: "${ISO_FILE:?Error: required env var not set}"
+      STORAGE_DIR: "${STORAGE_DIR:?Error: required env var not set}"
+    devices:
+      - /dev/kvm
+    cap_add:
+      - NET_ADMIN
+    ports:
+      - 8006:8006
+      - 3389:3389/tcp
+      - 3389:3389/udp
+    stop_grace_period: 2m
+    volumes:
+      - ${ISO_FILE}:/custom.iso
+      - ${STORAGE_DIR}:/storage