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.yml

@@ -3,8 +3,7 @@ services:
     image: windows-local:latest
     container_name: windows
     environment:
-      VERSION: "win11e"
-      STORAGE: "${STORAGE:?Error: required env var not set}"
+      STORAGE_DIR: "${STORAGE_DIR:?Error: required env var not set}"
     devices:
       - /dev/kvm
     cap_add:
@@ -15,15 +14,4 @@ services:
       - 3389:3389/udp
     stop_grace_period: 2m
     volumes:
-      - ${STORAGE}:/storage
-
-  prepare-golden-image:
-    extends:
-      service: windows
-    profiles:
-      - prepare-golden-image
-    environment:
-      ISO: "${ISO:?Error: required env var not set}"
-    volumes:
-      - ${ISO}:/custom.iso
-      - ${STORAGE}:/storage
+      - ${STORAGE_DIR}:/storage