feat(iso): add --clean-cache option to build-in-container.sh

Removes all cached build artifacts: Go cache, NVIDIA/NCCL/cuBLAS
downloads, lb package cache, and live-build work dir. Use before
a clean rebuild or when switching Debian/kernel versions.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/builder/build-in-container.sh

@@ -11,6 +11,7 @@ BUILDER_PLATFORM="${BEE_BUILDER_PLATFORM:-linux/amd64}"
 CACHE_DIR="${BEE_BUILDER_CACHE_DIR:-${REPO_ROOT}/dist/container-cache}"
 AUTH_KEYS=""
 REBUILD_IMAGE=0
+CLEAN_CACHE=0
 
 . "${BUILDER_DIR}/VERSIONS"
 
@@ -28,14 +29,31 @@ while [ $# -gt 0 ]; do
             AUTH_KEYS="$2"
             shift 2
             ;;
+        --clean-cache)
+            CLEAN_CACHE=1
+            shift
+            ;;
         *)
             echo "unknown arg: $1" >&2
-            echo "usage: $0 [--cache-dir /path] [--rebuild-image] [--authorized-keys /path/to/authorized_keys]" >&2
+            echo "usage: $0 [--cache-dir /path] [--rebuild-image] [--authorized-keys /path/to/authorized_keys] [--clean-cache]" >&2
             exit 1
             ;;
     esac
 done
 
+if [ "$CLEAN_CACHE" = "1" ]; then
+    echo "=== cleaning build cache: ${CACHE_DIR} ==="
+    rm -rf "${CACHE_DIR:?}/go-build" \
+           "${CACHE_DIR:?}/go-mod" \
+           "${CACHE_DIR:?}/tmp" \
+           "${CACHE_DIR:?}/bee" \
+           "${CACHE_DIR:?}/lb-packages"
+    echo "=== cleaning live-build work dir: ${REPO_ROOT}/dist/live-build-work ==="
+    rm -rf "${REPO_ROOT}/dist/live-build-work"
+    echo "=== done, caches cleared ==="
+    exit 0
+fi
+
 if ! command -v "$CONTAINER_TOOL" >/dev/null 2>&1; then
     echo "container tool not found: $CONTAINER_TOOL" >&2
     exit 1