diff --git a/tools/osbuilder/Makefile b/tools/osbuilder/Makefile
index b70ca7a20a..a35139c5a9 100644
--- a/tools/osbuilder/Makefile
+++ b/tools/osbuilder/Makefile
@@ -9,7 +9,7 @@ ROOTFS_BUILDER := $(MK_DIR)/rootfs-builder/rootfs.sh
 INITRD_BUILDER := $(MK_DIR)/initrd-builder/initrd_builder.sh
 IMAGE_BUILDER  := $(MK_DIR)/image-builder/image_builder.sh
 
-DISTRO                := centos
+DISTRO                ?= centos
 BUILD_METHOD          := distro
 BUILD_METHOD_LIST     := distro dracut
 AGENT_INIT            ?= no
diff --git a/tools/osbuilder/rootfs-builder/rootfs.sh b/tools/osbuilder/rootfs-builder/rootfs.sh
index c058c257a1..44fdab42ea 100755
--- a/tools/osbuilder/rootfs-builder/rootfs.sh
+++ b/tools/osbuilder/rootfs-builder/rootfs.sh
@@ -326,7 +326,11 @@ build_rootfs_distro()
 		trap error_handler ERR
 	fi
 
-	mkdir -p ${ROOTFS_DIR}
+	if [ -d "${ROOTFS_DIR}" ] && [ "${ROOTFS_DIR}" != "/" ]; then
+		rm -rf "${ROOTFS_DIR}"/*
+	else
+		mkdir -p ${ROOTFS_DIR}
+	fi
 
 	# need to detect rustc's version too?
 	detect_rust_version ||
@@ -369,6 +373,8 @@ build_rootfs_distro()
 
 		docker_run_args=""
 		docker_run_args+=" --rm"
+		# apt sync scans all possible fds in order to close them, incredibly slow on VMs
+		docker_run_args+=" --ulimit nofile=262144:262144"
 		docker_run_args+=" --runtime ${DOCKER_RUNTIME}"
 
 		if [ -z "${AGENT_SOURCE_BIN}" ] ; then